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CIHM/ICMH 

Microfiche 

Series. 


CIHM/ICMH 
Collection  de 
microfiches. 


Canac':  i ;  Institute  for  Historical  Microreproductions  /  Institut  Canadian  de  microreproductions  historiques 


L-_ 


Technical  and  Bibliographic  Notes/Notes  techniqu  js  et  bibliographiques 


The  Institute  has  attempted  to  obtain  the  best 
original  copy  available  for  filming.  Features  of  this 
copy  which  may  be  bibliographically  unique, 
which  may  alter  any  of  the  images  in  the 
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the  usual  method  of  filming,  are  checket'-  below. 


L'Institut  a  microfilm*  le  meilleur  exemplaire 
qu'il  lui  a  6t6  possible  de  se  procurer.  Les  details 
de  cet  exemplaire  qui  sont  peut-dtre  uniques  du 
point  de  vue  bibliographique,  qui  peuvent  modifier 
une  imago  reproduite,  ou  qui  peuvent  exiger  une 
modification  dans  la  m^thode  normale  de  filmage 
sont  indiquds  ci-dessous. 


□    Coloured  covers/ 
Cou  /erture  de  couleur 

□    Covers  damaged/ 
Couverture  endommagee 

□    Covers  restored  and/or  laminated/ 
Couverture  restaur6e  et/ou  pellicul6e 


D 
D 
D 
D 
□ 


D 


Cover  title  missing/ 

Le  titre  de  couverture  manque 

Coloured  maps/ 

Cartes  g6ographiques  en  couleur 

Coloured  ink  (i.e.  other  than  blue  or  black)/ 
Encre  de  couleur  (i.e.  autre  qub  hieue  ou  noira) 

Coloured  plates  and/or  illustrations/ 
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Bound  with  other  material/ 
Relie  avec  d'autres  documents 

Tight  binding  may  cause  shadows  or  distortion 
along  interior  margin/ 

La  reliure  serree  peut  causer  de  I'ombre  ou  de  la 
distortion  le  long  de  la  marge  int6rieure 

Blank  leaves  added  during  restoration  may 
appear  within  the  text.  Whenever  possible,  theie 
have  been  omitted  from  filming/ 
II  se  peut  que  certaines  pages  blanches  ajoutdes 
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pas  et*  film*es. 


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Pages  restaur-jes  et/o     pelliculees 


Pages  discoloured,  stained  or  foxed/ 
Pages  d*color6es,  tachetdes  ou  piqu^es 


I      I    Kages  detached/ 


\A 


n 


Pages  d*tach6e5 

Showthrough/ 
Transparence 


□    Quality  of  print  varies/ 
Qualite  in*gale  de  i'impression 

□    Includes  supplementary  material/ 
Compreitd  du  materiel  supplementaire 

□    Only  edition  available/ 
Seule  Edition  disponible 


Pages  wholly  or  partially  obscured  by  eirata 
slips,  tissues,  etc.,  have  been  refilmed  to 
ensure  the  best  possible  image/ 
Les  pages  totalement  ou  pariiellement 
obscurcies  par  un  feuillet  d'errata,  une  pclure, 
etc.,  ont  6t6  film6es  S  nouveau  de  ♦aeon  d 
obtenir  la  meilleure  image  possible. 


D 


Additional  comments:/ 
Commentaires  suppl6mentaires: 


This  item  is  filmed  at  the  reduction  ratio  checked  below/ 

Ce  document  est  film*  au  taux  de  r6duction  indiqu*  ci-dessous. 

10X  14X  18X  22X 


7 


12X 


16X 


20X 


26X 


30X 


n 


24X 


28X 


32X 


The  copy  filmed  here  has  been  reproduced  thanks 
to  the  generosity  of: 

National  Library  of  Canada 


L'exemplaire  film6  fut  reproduit  grdce  d  la 
g6n6rositd  de: 

Bibliothdque  nationale  du  Canada 


The  images  appearing  here  are  the  best  quality 
possible  considering  the  condition  and  legibility 
of  the  original  copy  and  in  keeping  with  the 
filming  contract  specifications. 


Original  copies  in  printed  paper  covers  are  filmed 
beginning  with  th?  front  cover  and  ending  on 
the  last  page  with  a  printed  or  illustrated  impres- 
sion, or  the  back  cover  when  appropriate.  All 
other  original  copies  are  filmed  beginning  on  the 
first  page  with  a  printed  or  illustrated  impres- 
sion, and  ending  on  the  last  page  with  a  printed 
or  illustrated  impression. 


The  last  recorded  frame  on  each  microfiche 
shall  contain  the  symbol  — ►  (meaning  "CON- 
TINUED"), or  the  symbol  V  (meaning  "END"), 
whichever  applies. 

Maps,  plates,  charts,  etc.,  may  be  filmed  at 
different  reduction  ratios.  Those  too  large  to  be 
entirely  included  in  one  exposure  are  filmed 
beginning  in  the  upper  left  hand  corner,  left  to 
right  and  top  to  bottom,  as  many  frames  as 
required.  The  following  diagrams  illustrate  the 
method: 


Les  images  suivantes  ont  6t6  reproduites  avec  le 
plus  grand  soin,  compte  tenu  de  la  condition  et 
de  la  nettet^  de  l'exemplaire  film6,  et  en 
conformity  avec  les  conditions  du  contrat  de 
filmage. 

Les  exemploires  originaux  dont  la  couverture  en 
papier  est  imprim6e  sont  film6s  en  commengant 
par  le  premier  plat  et  en  terminant  soit  par  la 
dernidre  page  qui  comporte  une  empreinte 
d'impression  ou  d'illustration,  soit  par  le  second 
plat,  selon  le  cas.  Tou"  les  autres  e^emplaires 
originau  ..  sont  filmds  ^n  commenpant  par  la 
premidre  page  qui  comoorte  une  empreinte 
d'impression  ou  d'illustration  et  en  terminant  par 
la  dernidre  page  qui  comporte  une  telle 
empreinte. 

Un  des  symboles  suivants  apparaitra  sur  la 
dernidre  image  de  cheque  microfiche,  selon  le 
cas:  le  symbole  — ►  signifie  "A  SUIVRE",  le 
symbole  V  signifie  "FIN". 

Les  cartes,  planches,  tableaux,  etc.,  peuvent  etre 
filmds  d  des  taux  de  reduction  diffdrents. 
Lorsque  le  document  est  trop  grand  pour  etre 
reproduit  en  un  seul  clichd,  il  est  film6  d  partir 
de  Tangle  sup§rieur  gaucha,  de  gauche  h  droite, 
3t  de  haut  en  bas,  en  prenant  le  nombre 
d'images  n6cessaire.  Les  diagrammes  suivants 
illustrent  la  mdthode. 


i 

2 

3 

4 

5 

6 

BIOLOGY 


...v ■;,■•■.    tv 


INTRODUCTION     . 


TO  THE 


STUDY  OF  BIOLOGY.  . 


BY 

H.    ALLEYNE   NICHOLSON, 

M.  D.,  D.  Sc,   M.A.,   Ph.D.,  F.R.S.  E.,   F.G.S.,  etc., 

PROFESSOR   OF  NATURAL  HISTORY  AND   BOTANY  IN   UNIVERSITY  COLLEGE,  TORONTO, 

FORMERLY  LECTURER  ON    NATURAL  HISTORY   IN  THE  MEDICAL  SCHOOL 

OF  EDINBURGH,   ETC.,    ETC.;    AUTHOR  OF   "  TEXT-BOOK  OF 

GEOLOGY,"     "TEXT-BOOK    OF    ZOOLOGY," 

"MANUAL  OF  ZOOLOGY." 


NEW   YORK: 
D.    APPLETON    AND    COMPANY, 

549    &    55:    BROADWAY.  .,,... 

1872. 


PREFACE. 


The  present  work  is  based  chiefly  upon  the  Intro- 
duction to  the  author's  '  Manual  of  Zoology,'  much 
of  which  is  given  here  in  an  unaltered  form.  A  con- 
siderable portion,  however,  of  the  Introduction  has 
been  here  recast,  whilst  Vully  two-thirds  of  the  work 
consists  of  new  matter.  Illustrations  also  have  been 
introduced  wherever  such  appeared  to  be  necessary. 

Many  important  subjects  have,  of  course,  been  ne- 
cessarily treated  very  superficially,  or  altogether  omit- 
ted, as  unsuitable  for  a  merely  elementary  work.  It 
is  hoped,  however,  that  most  of  those  subjects  are 
touched  upon,  a  knowledge  of  which  would  be  useful 
to  the  student  of  living  or  extinct  forms  of  life,  or 
to  the  general  reader. 

Toronto,  Canada,  Z>^^^*w^^  4,  1871. 


,A'''V 


m 


CONTENTS. 


CHAPTER  I. 

PAGR 

Definition  of  Biology — Differences  between  dead  and  living  bodies 
— Nature  and  conditions  of  life — Physical  basis  of  life-  'Pro- 
toplasm— Connection  between  life  and  the  matter  of  life — Or- 
ganisation —  Light  —  Air  —  Temperature  —  Death  —  Use  of 
the  term  "vital  force," l-l8 


CHAPTER  II. 

Differences  bet»veen  animals  and  plants — Regnum  Protibticum  of 
Ilaeckel — Higher  plants  distinguished  from  higher  animals — 
Comparison  of  the  lower  animals  with  the  lower  plants — 
Form — Internal  structure— Chemical  composition— Power  of 
locomotion — Nature  of  the  food, ^9'^$ 


CHAPTER  III. 

Differences  between  different  organisms — Morphology — Physiology 
— Specialisation  of  tunctions — Morphological  type — Synopsis 
of  the  main  divisions  of  the  animal  and  vegetable  kingdoms,  26-43 


CHAPTER  IV. 

Analogy  —  Homology — Serial  Homology  —  Lateral  Homology — :_  _ 
Homogeny  and   Homoplasy  —  Homomorphism  —  Mimicry- 
Correlation  of  growth, 44-5 


viii  CONTENTS. 

CHAPTKR  V. 

Principles  of  classification  — Definition  of  Sptjries — Genus — I'amily 
— Order — Class — Sub-kingdom  -  -Impossibility  of  a  linear  clas- 
sification,  $^'^3 

CHAPTER  VI. 

Elementary  chemistry  of  animals  ailfl  plants  —  Chemistry  of 
animals  —  Fats  -Albumen —  Fibrine  —  Caseine  —  Proteinc  of 
Mulder — Chemistry  of  vegetables — Starch — Cellulose — Sugar 
—  Albuminous  compounds  of  plants,        ....       64-69 

CHAPTER  VII. 

Elementary  structure  of  living  bodies — Protoplasm  or  Uioplr.sm — 
Molecules  —  Cells — The  cell-wall  —  The  ccU-conlcnts  —  The 
nucleus — Cell-multiplication, 70-76 

CHAPTER  VIII. 

Physiological  functions  of  animals  and  plants — Animal  and  vege- 
table functions — Un'cellular  plants — Foraminifera— Vital  force 
as  manifested  in  the  digestive  process  of  plants,        .         .       77-83 

CHAPTER    IX. 

General  phenomena  of  nutrition — Assimilation— Death— Growth — 
Development  —  Transformation  —  Metamorphosis  —  Law  of 
Quatrefages — Provisional  organs  of  young  animals — Absence 
of  sexual  reproduction  in  larval  form?— Von  Baer's  law  of 
development — Retrograde  development,  .         .         .         84-96 

CHAPTER  X. 

Reproduction— Sexual  reproduction — Non-sexual  reproduction — ■ 
General  phenomena  of  gemmation  and  fission — Gemmation  in 
ihe  Foraminifera — Gemmation  in  the  Sea-mat — Gemmation  • 
in  Hydra — Fission  in  Paramoecium — Definition  of  the  zoologi- 
cal individual — Zooids— Internal  gemmation  of  Polyzoa — Al- 
ternation of  generations— Reproduction  of  Ilydractinia — Re- 
production of  Clytia — Structure  of  free  medusiform  zooids — 
Reproduction  of  the  Lucemarida— Parthenogenesis— Of  Aph- 
ides—Of Bees— Law  of  Quatrefages — Antagonism  between 
sexual  reproduction  and  nutrition,  ....       97-118 


CONTENTa 


IX 


CHAPTER  XI. 

Reproduction  in  plants— Gemmation  and  fission— Resemblances  be 
tween  plants  and  Ilydroidzoophytcs— Reproduction  of  Angio- 
spermous  flowering  plaats— Reproduction  of  ferns,  .     1 19-126 

CHAPTER  XH. 

Spontaneous  generation— Development  of  living  beings  in  organic 
infusi*-  :£— Experiments  of  Dr  Bastian,  .         .         .         .     127-133 

CHAPTER  XIII. 

Origin  of  species— Doctrine  of  Special  Crerttion— Doctrine  of  Evol- 
ution—Views of  Lamarck— The  Darwinian  hypothesis  — 
Theory  of  natural  selection — Sexual  selection— Leading  objec- 
tions to  the  theory  of  the  evolution  of  species  by  natural 
selection 134- H3 

CHAPTER  XIV. 

Distribution  in  space— Geographical  distribution—  Zoological  pro- 
vinces—Bathymetrical  distribution— Discoveries  in  the  deep 
sea— Conditions  of  life  in  the  deep-sea  animals,      .        .     144-150 


CHAPTER  XV. 

Distribution  in  time — Laws  of  geological  distribution — Chief  divi- 
sions of  ^he  stratifiicd  series — Contemporaneity  of  strata— Geo- 
logical continuity— Imperfection  of  the  palreontological  re- 
cord,      151-160 


* 


LIST   OF    ILLUSTRATIONS. 


riG. 
I. 

2. 

3- 
4- 
S- 


7- 
8. 

9- 

lO. 

II. 

12. 

13- 
14. 

15- 
16. 

17- 
18. 
19. 
20. 
21. 
22. 

23. 
24. 

25. 
26. 
27. 
28. 


Nonionina  and  Gromia,     .... 

Wheel-animalcule,  .....  ^ 

Ciliated  spores  of  Plants,  Volvox  globator,  and  Euplotes  Charon 
Amoeba,       ...... 

Actinia  mesembryanthcmum,  and  diagrammatic  section  of  the 
same,       ...... 

Diagrammatic  section  of  a  Whelk, 

Gregarine,  Rhizopod,  and  Infusorian, 

Hydra  vulgaris,  and  diagrammatic  section  of  the  same, 

Holcthurian  and  larva,       .... 

Diagram  of  Annulose  animal. 

Section  of  a  Cephalopod,    .... 

Skeleton  of  the  Common  Perch,     . 

Fore-limb  of  Man,  Fore-leg  of  Dog,  and  Wing  of  Bird, 

Fairy  Shrimp,         ..... 

Centipede,  ...... 

Arm  of  Chimpanzee,  .... 

Leg  of  Chimpanzee, 

Phyllium  siccifolium,         .... 

Yeast-plant,  ..... 

Cells  of  notochord  of  Lamprey,     . 

Ovum  of  Ascaris  nigovenosa, 

Germinating  cells  of  Yeast-plant,  ... 

Metamorphoses  of  Butterfly,  .... 

Young  of  water-breathing  Gasteropod  and  adult  Pteropod, 
Young  of  Achtheres,  and  adult  Lerncea,    . 
Diagram  of  Foramini/era,  .  ,  . 

Flustra  hispida,      .  .  .  ^  . 

ParamcEcium  multiplying  by  fission,         .  , 


PACK 

13 
IS 

21 
29 

30 
31 

3S 
36 
37 
38 
40 

41 

45 

46 

46 

47 

47 

S3 
72 

73 
IS 
76 

90 

95 
96 

99 
100 
xox 


Xll 


LIST  OF   ILLUSTRATIONS. 


29.  Group  of  Hydraetinia  tchinata,  with  gonophorcs, 

30.  Trophosome  of  Clytia  yohnstoni, 
3t.  Free  Medusoid  of  Clytia  Johnstoni, 

32.  Development  of /fwr^/m,  . 

33.  Generative  zooid  of  Chrysaora, 

34.  Bean  Aphis,  .... 

35.  Male  organs  and  pollen  of  Flowering  Plants, 

36.  Female  organs  of  Flowering  Plants, 

37.  Fructifying    Frond,   Spore-cases,   Spore,   and 

Fern,       ..... 

38.  Molecules  and  bacteria  of  organic  infusions, 

39.  Ideal  section  of  the  Crust  of  the  Earth     . 


3, 

*05 

roS 

•                                                   •                                                   • 

109 

•                                                   •                                                   • 

no 

•                                                   •                                                   • 

III 

•                                                    •                                                   • 

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•                                                    •                                                   • 

Z2I 

• 

122 

Prothallus  of  a 

•           •           • 

125 

•           •           • 

128 

•           •           • 

»S3 

ELEMENTS    OF    BIOLOGY. 


CHAPTER    I. 


DEFINITION     OF     BIOLOGY. 


All  nafuial  objects  admit  of  an  obvious  separation  into 
two  primary  groups,  according  as  they  are  dead  or  alive — 
according  as  they  exhibit  no  phenomena  except  such  as 
can  readily  be  referred  to  the  working  of  known  physical 
and  chemical  laws,  or  as  they  present,  in  addition,  the  phe- 
nomena which  we  are  accustomed  to  group  together  under 
the  name  of  "  vital."  The  studies  which  occupy  themselves 
with  dead  bodies  concern  the  physicist,  the  chemist,  the 
geologist,  and  the  mineralogist.  The  study  of  living  be- 
ings, irrespective  of  the  exact  nature  and  position  of  these, 
is  the  province  of  Biology  (Gr.  bios^  life ;  logos^  a  discourse). 
All  living  beings,  however,  may  be  divided  into  the  two 
kingdoms  of  animals  and  plants,  the  study  of  the  former 
constituting  the  department  of  Zoology,  whilst  Botany  is 
exclusively  concerned  with  the  latter.  In  accordance  with 
this  division,  Biology  splits  up  into  the  kindred  sciences  of 
Zoology  and  Botany,  and  properly  includes  both  of  these  in 
all  their  details.  Here,  however,  nothing  more  is  aimed  at 
than  the  presenting  to  the  student  in  a  concise  form  some 


2  '  ELEMENTS  OF  BIOLOGY. 

of  the  leading  principles  upon  v/hich  the  sciences  of  Zoology 
and  Botany  are  based.  With  this  view,  technicalities  will 
be  as  far  as  may  be  avoided ;  and  on  all  matters  which  are 
still  undecided  the  evidence  on  both  sides  will  as  far  as 
possible  be  given,  so  that  the  reader  may  be  enabled  to 
form  his  own  judgment  as  to  the  questions  at  issue. 

DIFFERENCES    BETWEEN    DEAD    AND   LIVING   BODIES. 

In  marking  out  the  boundaries  which  limit  the  province 
of  Biology,  the  first  point  is  obviously  to  arrive  at  a  clear 
conception  as  to  the  differences  M'hich  separate  all  living 
bodies  from  those  that  are  dead.  The  leading  char-^cters 
by  which  living  bodies  are  distinguished  from  uead  bodies 
may  be  summed  up  as  follows :  Firstly^  Every  living  body 
possesses  the  power  of  taking  into  its  interior  certain  ma- 
terial foreign  to  those  composing  its  own  substance,  and 
of  (  onverting  these  into  the  materials  of  which  its  body 
is  bu '^  up.  This  constitutes  the  process  of  "assinfllation," 
and  11  in  virtue  of  this  that  living  bodies  grow.  In 
all  cases  alike,  the  materials  to  be  assimilated  are  taken 
into  the  interior  of  the  body,  and  the  process  of  growth  is, 
therefore,  one  depending  upon  the  "  intussusception "  of 
foreign  matter  in  contradistinction  to  its  mere  addition 
from  the  outside,     j;;^'  f"i    ^  ;       :.  j         7  .    -  •  :  \  y  r?  •  »% 

When,  on  the  other  hand,  dead  bodies  increase  in  size, 
as  crystals  do,  the  increase  is  produced  simply  by  the  addi- 
tion of  fresh  particles  from  the  exterior,  or,  as  it  is  techni- 
cally called,  by  the  "  accretion  "  of  matter.  This  process 
cannot  properly  be  considered  as  one  of  "  growth,"  as 
being  wholly  destitute  of  the  essential  element  of  a  pre- 
vious "  assimilation." 

Secondly,  All  the  actions  of  living  beings  are  ac-companied 
by  a  corresponding  destruction  of  the  matter  by  which 
these  actions  are  manifested.  In  other  words,  partial 
death  is  a  constant  accompaniment  of  life ;  and  the  inces- 
sant loss  of  substance  caused   by  vital   action  has  to  be 


DEAD   AND   LIVING   BODIHS.  3 

compensated  for  by  the  simultaneous  assimilation  of  an 
equivalent  amount  of  fresh  matter. 

Thirdly^  If  our  observation  be  continued  for  a  sufficient 
length  of  time,  every  living  Dody  has  the  power  of  jepro- 
ducing  its  like.  That  is  to  say,  every  living  body  has  the 
power,  directly  or  indirectly,  of  giving  rise  to  minute  germs, 
which,  under  proper  conditions,  will  be  developed  into  the 
likeness  of  the  parent. 

Fourthly^  Dead  bodies  are  subject  to  the  physical  and 
chemical  forces  of  the  universe,  and  have  no  power  of 
suspending  these  forces,  or  modifying  their  action,  even  for 
a  limited  period.  On  the  other  hand,  living  bodies,  whilst 
subject  to  the  same  forces,  are  the  seat  of  something  in 
virtue  of  which  they  can  override,  suspend,  or  modify  the 
actions  of  the  physical  and  chemical  forces  by  which  dead 
bodies  are  exclusively  governed.  Dead  matter  is  com- 
pletely passive,  unable  to  originate  motion,  and  equally  un- 
able to  arrest  it  when  once  originated.  Living  matter,  so 
long  as  it  is  living,  is  the  seat  of  energy^  and  can  overcome 
the  primary  law  of  the  inertia  of  matter.  However  humble 
it  may  be,  and  even  if  permanently  rooted  to  one  place, 
every  living  body  possesses,  in  some  part  or  other,  or  at 
some  period  of  its  existence,  the  power  of  independent  and 
spontaneous  movement — a  power  possessed  by  nothing 
that  is  dead.  Similarly,  the  chemical  forces,  which  work 
unresisted  amongst  the  particles  of  dead  matter,  are  in  the 
living  organism  directed  harmoniously  to  given  ends,  their 
action  regulated  under  definite  laws,  and  their  natural  work- 
ing often  strikingly  modified,  or  even  temporarily  suspended, 
and  this  as  effectually  and  as  perfectly  in  the  humblest  as 
in  the  highest  of  created  beings. 

As  a  result  of  this,  dead  bodies  exhibit  nothing ,  but  re- 
actions, and  these  purely  of  a  physical  and  chemical  na- 
ture, whilst  they  show  no  tendency  to  pass  through  periodi- 
cal changes  of  state.  On  the  other  hand,  living  bodies  exhibit 
distinct  actions,  and  are  pre-eminently  characterised  by  their 


4  ELEMENTS  OF  BIOLOGY. 

tendency  tc  pass  through  a  series  of  cyclical  changes,  which 
follow  one  another  in  a  regular  and  determinate  sequence. 

The  above  points  are  the  leading  characters  by  which 
living  bodies  are  fundamentally  separated  from  dead  matter. 
There  are,  however,  a  few  subordinate  points  in  which 
some  or  all  living  bodies  differ  from  those  which  are  dead : — 

a.  Chemical  Composition. — Dead  bodies  are  composed  of 
numerous  elements,  which  exist  either  in  an  uncombined 
condition,  or  in  a  state  of  union.  The  combinations  of 
these  elements  may  be  said  to  be  naturally  in  a  state  of 
stable  equilibrium,  and  they  show  no  tendency  to  spon- 
taneous decomposition.  Further,  the  combining  elements 
unite  with  one  another  in  low  combining  proportions,  and 
the  resulting  compounds  for  the  most  part  consist  of  no 
more  than  two  or  three  elements.  ■  - 

Living  bodies,  on  the  other  hand,  are  composed  of  few 
chemical  elements,  and  these  are  almost  always  in  a  state 
of  combination.  Furthermore,  the  combinations  are  always 
complex,  consisting  of  three  or  four  elements,  and  these 
elements  are  united  with  one  another  in  high  combining 
proportions.  Finally,  the  chemical  compounds  of  living 
bodies  are  invariably  characterised  by  the  presence  of 
water,  and  are  prone  to  spontaneous  decomposition.  Thus, 
the  great  organic  compound,  albumen,  is  composed  of  144 
atoms  of  carbon,  no  of  hydrogen,  18  of  nitrogen,  42  of 
oxygen,  and  two  atoms  of  sulphur.  Iron,  however,  exists 
in  the  blood,  possibly  in  its  elemental  condition,  and  cop- 
per has  been  detected  in  the  liver  of  certain  of  the  Mam- 
malia, and  largely  in  the  colouring-matter  of  the  feathers  of 
certain  birds.  It  is  to  be  remembered,  also,  that  certain 
mineral  salts,  well  known  as  occurring  in  dead  nature,  are 
apparently  absolutely  indispensable  to  Jiving  bodies,  at  any 
rate  as  a  general  rule.  Living  bodies,  therefore,  whilst 
certainly  presenting  us  with  a  peculiar  group  of  chemical 
compounds,  are  to  a  certain  extent  built  up  of  substances 
which  commonly  occur  dissociated  from  vitality. 


1 


NATURE   AND  CONDITIONS  OF    MFE.  5 

If.  Arravgement  of  parts, — Dead  bodies,  when  unmixed,  are 
composed  of  an  aggregation  of  similar  and  homogeneous  parts 
which  bear  no  definite  and  fixed  relations  to  one  another. 

Living  bodies,  on  the  other  hand,  are  in  the  great  majority 
of  cases  composed  of  dissimilar  and  heterogeneous  parts, 
the  relations  of  which  amongst  themselves  are  more  or  less 
definite.  In  other  words,  most  living  bodies  are  "  organ- 
ised," being  composed  of  separate  parts  or  "  organs,"  which 
have  certain  definite  functions  in  the  general  economy.  It 
must,  ho\vf»vpr,  be  borne  in  mind,  that  organisation,  though 
in  the  vast  proportion  of  cases  a  concomitant  of  vitality,  is 
not  necessarily  present  in  living  bodies.  Some  living  beings 
(such  as  the  minute  organisms  known  as  the  Foraminifera)  ex- 
hibit no  distinct  parts  or  organs,  and  cannot  therefore  be  said 
to  be  "organised"  in  any  proper  sense  of  the  term,  whilst  they, 
nevertheless,  exhibit  all  the  essential  phenomena  of  vitality. 

c.  Form. — Dead  bodies  are  either  of  no  definite  shape — 
when  they  are  said  to  be  "  amorphous  " — or  they  are  crys- 
talline, in  which  case  they  are  almost  invariably  bounded 
by  straight  lines  and  plane  surfiices.  Living  bodies  are 
almost  always  of  a  definite  shape,  presenting  convex  and 
concave  surfaces,  and  being  bounded  by  curved  lines. 
Some  living  bodies,  however,  cannot  bd  said  to  have  any 
fixed  form,  but  are  extremely  mutable  in  figure.  In  no 
case,  however,  could  such  be  confounded  with  either  the 
amorphous  or  the  crystalline  forms  of  dead  matter. 

NATURE   AND   CONDITIONS   OF   LIFE. 

Life  has  been  variously  defined  by  different  writers. 
Bichat  defines  life  as  *'  the  sum  total  of  the  forces  which 
resist  death;"  Treviranus,  as  "the  constant  uniformity  of 
phenomena  with  diversity  of  external  influeiices ; "  Duges, 
as  "  the  special  activity  of  organi?'  d  bodies;"  and  Beclard, 
as  "organisation  in  action."  i^..  these  definitions,  how- 
ever, are  more  or  less  objectionable,  either  because  they 
really  express  nothing,  or  because  the  assumption  under- 


6  ELEMENTS  OF   niOLOGY. 

lies  them  that  life  is  inseparably  connected  with  organisa- 
t'on.  More  recently  attempts  have  been  made  to  prove 
that  lite  is  merely  a  form  of  energy  or  motion,  in  which  case 
no  difticully  should  be  found  in  giving  it  an  exact  defini- 
tion. In  the  meanwhile,  however,  this  view  certainly  can- 
not be  said  to  have  been  satisfactorily  proved,  and  it  does 
not  appear  that  any  rigid  definition  of  life  is  possible.  We 
may  therefore  employ  the  name  life  as  a  collective  term 
for  the  tendency  exhibited  by  certain  forms  of  matter,  under 
certain  conditions,  to  pass  through  a  series  of  changes  in  a 
more  or  less  definite  and  determinate  sequence. 

As  regards  the  conditions  under  which  alone  life  or  vital 
activity  can  be  manifested,  we  have  to  consider  two  sets  of 
conditions  :  the  intrinsic  or  indispensable  conditions,  with- 
out which  no  vital  phenomena  are  possible;  and  the  extrin- 
sic conditions,  which  are  generally  present,  but  which  do 
not  appear  to  be  actually  essential  tD  living  beings.  •  Under 
the  first  head,  we  have  only  to  consider  the  presence  of  a 
"  physical  basis  ; "  und^r  the  second  head,  we  may  briefly 
look  to  the  presence  of  organisation,  light  and  air,  and  the 
necessity  for  a  certain  temperature. 

a.  Protoplasm. — The  first  of  the  questions  as  to  the  con- 
ditions of  life  which  it  is  necessary  to  consider,  is  whether 
the  phenomena  of  vitality  are  necessarily  associated  with 
any  particular  form  of  matter,  or  with  any  special  "  physical 
basis,"  as  it  has  been  aptly  termed.  The  answer  to  this 
question  may  with  little  hesitation  be  given  in  the  affirma- 
tive. It  does  not  at  all  appear  that  the  phenomena  of  life 
can  be  manifested  by  any  and  every  form  of  matter ;  and  a 
very  little  reflection  ought  to  convince  us  that  it  would  be 
very  surprising  if  the  reverse  of  this  were  the  case.  There 
is  no  physical  or  chemical  force  which  can  be  rendered 
manifest  to  us  by  all  and  sundry  forms  of  matter,  and  it 
would  be  indeed  remarkable  if  the  case  were  otherwise  with 
the  forces  of  the  living  organism.  When,  for  example,  we 
say  that  certain  forms  of  matter,  such  as  the  metals,  are 


NATURE   AND  CONDITIONS  OK   LIFE. 


conductors  of  electricity,  ant!  certain  other  forms,  such  as 
glass,  are  non  conductors,  we  are  in  truth  saying  that  elec- 
tricity requires  fc-  its  manifertiition  a  certain  "  physical 
basis."  Upon  merc'y  tlieoi^tical  grounds,  therefore,  we 
might  have  assumed  the  existence  of  a  **  matter  of  life,"  or  • 
a  physical  basis  absolutely  necessary  for  the  manifestation 
of  vital  phenomena.  This  physical  basis  of  life  is  known 
by  the  now  notorious  name  of  "  protoplasnj,"  or,  as  it  is 
better  termed  by  Dr  Beale,  "  bioplasm."     ■     .  : 

As  regards  its  nature,  protoplasm,  though  capable  of 
being  built  up  into  the  most  complex  structures,  does  not 
necessarily  exhibit  anything  which  can  be  looked  upon  as 
organisation  or  differentiation  into  distinct  parts  ;  and  its 
chemical  composition  is  the  only  constant  which  can  be 
approximately  stated.  It  consists,  namely,  of  carbon,  oxy- 
gen, nitrogen,  and  hydrogen,  united  into  a  proximate  com- 
pound to  which  Mulder  applied  the  name  of  "proteine," 
and  which  is  very  nearly  identical  with  albumen  or  white-of- 
egg.  It  further  appears  probable  that  all  forms  of  proto- 
plasm can  be  made  to  contract  by  electricity,  and  "are 
liable  to  uiidergo  that  peculiar  coagulation  at  a  temperature 
of  4o°-5o''  centigrade,  which  has  been  called  *  heat-stiffen- 
ing ' "  (Huxle").  Protoplasm,  therefore,  may  be  regarded 
as  a  general  term  for  all  forms  of  albuminoid  matter ;  and, 
in  this  general  sense,  we  may  safely  assert  that  protoplasm 
is  the  "  physical  basis  "  of  life  ;  or,  in  other  words,  that  vital 
phenomena  cannot  be  manifested  except  through  the  me- 
dium of  a  protoplasmic  body.  It  is  to  be  borne  in  mind, 
however,  that  it  has  not  yet  been  shown  that  all  the  forms  of 
matter  which  we  include  under  the  conveniently  loose  term 
of  "  protoplasm,"  have  a  constant  and  undeviating  chemical 
composition.  It  must  also  be  remembered  that  there  a  e 
certain  other  substances,  such  as  some  of  the  minel^ai  salts, 
which,  though  only  present  in  small  quantity,  nevertheless 
appear  to  be  absolutely  essential  to  the  maintenance  of  life, 
at  the  same  time  that  their  exact  use  is  not  at  present  known. 


8  KLnMENTS  OF   UlOI.Or.Y. 

It  seems  certain,  then,  that  no  body  is  capable  of  mani- 
festing the  marvellous  phenomena  of  life,  unless  it  be  com- 
posed of  some  form  or  other  of  albuminous  or  protoplasmic 
matteT.  We  know,  at  any  rate,  of  no  such  body  at  present, 
and  we  are  therefore  justified  in  asserting  that  the  presence 
of  an  albuminous  basis  is  an  essential  condition  of  vitality. 
Most  naturalists  probably  would  subscribe  to  this  state- 
ment ;  but  there  are  two  different  senses  in  which  it  would 
be  received.  Some  eminent  authorities  insist  that  albumin- 
ous matter  or  protoplasm  is  not  only  a  condition  ot  vitality, 
but  that  it  is  its  cause';  or,  in  other  words,  tnat  life  is  one 
of  the  properties  of  protoplasm.  It  is  asse/ted,  namely, 
that  life  is  the  result  of  the  combined  properties  of  the 
elements  which  form  albuminous  matter,  just  as  the  proper- 
ties of  water  are  the  resultant  of  the  combined  properties 
of  its  constituent  hydrogen  and  oxygen ;  and  it  is  alleged 
that  it  is  just  as  absurd  to  set  down  the  phenomena  of  life 
to  an  assumed  "  vital  force,"  as  it  would  be  to  ascribe  the 
properties  of  water  to  an  assumed  "  aquosity."  On  the 
other  hand,  equally  eminent  philosophers  would  assert  that 
the  view  just  mentioned  is  one  which  confounds  effect  with 
cause,  and  that  albuminous  matter  is  at  best  but  a  condition 
of  vitality,  just  as  the  presence  of  a  conductor  may  be  said 
to  be  an  esseVitial  condition  of  electricity.  The  question 
as  to  which  of  these  Iwo  opposing  views  has  most  in  its 
favour  is  one  of  sufficient  importance  to  warrant  a  brief 
exposition  of  the  grounds  upon  which  a  decision  may  be 
arrived  at. 

In  the  first  place,  when  we  come  to  sum  up  the  actual 
data  upon  which  such  a  decision  should  be  formed,  it  is 
clear  that  we  know  two  factors  only  of  the  case.  We  recog- 
nise certain  phenomena  which  we  call  "  vital,"  as  being 
exclusively  manifested  by  living  beings.  We  recognise, 
further,  that  these  phenomena  are  never  manifested  except 
by  certain  forms  of  matter,  or,  it  may  be,  by  but  a  single 
form  of  matter.    We  conclude,  therefore,  that  there  must  be 


NATURE   AND  CONDITIONS  OF   LIFK. 


an  intimate  connection  between  vital  phenomena  and  the 
'•  matter  of  life ; "  but  we  can  go  no  further  than  this,  and 
the  premisses  do  not  in  any  way  warrant  the  assertion  that 
life  is  the  result  of  living  matter,  or  one  of  its  properties. 
AVe  know  the  succession  of  phenomena,  but  we  know  no 
more,  and  it  is  not  possible  to  decide  dogmatically  which 
phenomenon  precedes  the  other  in  point  of  time.  It  is 
therefore  just  as  reasonable  to  believe  that  the  matter  of 
life  is  the  result  of  vital  forces  as  the  reverse ;  and,  as  far  as 
mere  logic  is  concerned,  neither  view  can  claim  the  smallest 
advantage  over  the  other. 

If  we  take  such  a  microscopic  animalcule  as  the  AtPir':bay 
or,  still  better,  one  of  the  yet  more  humble  organisms  which 
are  known  as  Foramini/cra,  we  are  presented  with  a  little 
speck  of  animal  matter,  a  little  particle  of  albumen,  almost 
or  quite  destitute  of  structure,  and  yet  exhibiting  all  the 
essential  phenomena  of  vitality.  Such  a  particle  of  living 
matter  is  undoubtedly  the  seat  of  certain  forces  which  render 
it  different  from  any  and  every  collocation  of  mere  dead 
particles.  Whether  we  call  these  forces  "  vital "  or  not 
matters  little ;  but  we  certainly  are  not  at  present  justified, 
by  any  evidence  in  our  hands,  in  asserting  that  they  are 
merely  a  form  of  energy  or  motion.  No  one  has  hitherto 
succeeded  in  demonstrating  how  any  form  or  any  combina- 
tion of  any  of  the  known  physical  or  chemical  forces  should 
produce  the  vital  phenomena  which  are  seen  to  occur  in  the 
albuminous  matter  of  even  the  most  humble  of  animals. 
Until  such  a  demonstration  can  be  brought  forward,  we  are 
not  only  justified,  but  we  are  bound,  to  look  at  the  forces  at 
work  in  living  matter  as  something  outside  and  beyond  the 
mere  physical  forces.  We  may  call  these  forces  "vital"  or  not, 
as  we  choose,  but  the  fact  will  either  way  remain  the  same. 

Again,  every  one  will  willingly  admit  that  all  compound 
substances  possess  certain  properties  which  are  the  result 
of  the  combined  properties  of  their  component  elements. 
Water,  for  example,  is  composed  of  hydrogen  and  oxygen. 


10  I'.I.KiMKNTS  OK   lUOUJGY. 

and  Its  properties  are  the  resultant  of  the  eoml  ined  proper- 
tics  of  these  two  gases.     It  is  a  definite  dicmical  compound, 
having  definite  and  constant  properties,  and  there  is  no 
kind  of  necessity  for  ascribing  the  i)ropcrties  of  water  to  any 
assumed  principle  of  "aquosity."     It  is  to  be  remembered, 
however,  that  there  is  only  one  kind  of  water,  and  its  pro- 
perties are  universally  the  same.     In  the  same  way,  alb    ni- 
nous  matter,  or  protoplasm,  is  a  chemical  compound  ».«ili.h 
unquestionably  possesses  certain  properties  as  the  result- 
ant of  the  combined  properties  of  its  component  elements. 
But  this  is  dead  protoplasm  of  which  this  is  true,  and  unless 
this  be  granted  it  is  difficult  to  see  ho*w  to  avoid  having  to 
deny  that  dead  protoplasm  can  exist  at  all.     It  is  conceiv- 
able— nay,  more,  it  is  one  of  the  splendid  possibilities  of  the 
future — that  the  chemist  should  sflcceed  in  forcing  the  ele- 
ments of  albuminous  matter  to  combine  with  one  another, 
and  thus  in   manufiicturing  protoplasm  artificially  in   the 
laboratory.    But  this  would  be  dead  albuminous  matter;  and 
it  is  wholly  inconceivable  that  the  utmost  advances  of  con- 
structive chemistry  should  ever  lead  to  the  manufacture  of 
living  protoplasm.      Dead  albuminous  matter  may  be  re- 
garded as  a  tolerably  definite  and  uniform  chemical  com- 
pound, and  its  properties  are,  beyond  doubt,  the  resultant 
of  those  of  its  component  elements.     Like  water,  therefore, 
dead  protoplasm   has  universally  the   same   physical   and 
chemical  properties.    Living  ;:rotoplasm,  on  the  other  hand, 
though  still  unchanged  in  chemical  composition  and  physi- 
cal characters,  exhibits  the  most  varied  properties,  accord- 
ing as  its  forms  enter  into  the  composftion  of  different  ani- 
mals.    If,  then,  we  are  to  ascribe  vital  phenomena  to  the 
inherent  constitution  of  living  matter — in  the  sense  that  the 
properties  of  water  are  those  of  its  component  gases — we 
are  left  to  account,  as  best  we  may,  for  the  utterly  immea- 
surable differences  between  the  vital  phenomena  of  a  man 
and  of  a  sponge,  both  of  which  may  be  regarded  as  com- 
posed fundamentally  of  the  same  materials. 


NATURK  AND  (ONDITIONS  oK    MIK. 


II 


The  more  philosophical  view,  then,  as  to  the  nature  of 
the  connection  between  life  and  its  niuierial  basis,  is  the  one 
which  regards  vitality  as  somcliiing  superadded  and  foreign 
to  the  matter  by  which  vital  phenomena  are  manifested. 
Protoplasm  is  essential  as  the  physical  medium  through 
which  vital  action  may  be  manifested  ;  just  as  a  conductor 
is  essential  to  the  manifestation  of  electric  phenomena,  or 
just  as  a  paint-brush  and  colours  are  essential  to  the  artist. 
Because  metal  conducts  the  electric  current,  and  renders  it 
perceptible  to  our  senses,  no  one  thinks  of  therefore  assert- 
ing that  electricity  is  one  of  the  inherent  proi)erties  of  a 
metal,  any  more  than  one  would  feel  inclined  to  assert  that 
tie  power  of  painting  was  inherent  in  the  camel's  hair  or  in 
the  dead  pigments.  Behind  the  material  substratum,  in  all 
cases,  is  the  active  and  living  force ;  and  we  have  no  right 
to  assume  that  the  force  i:eases  to  exist  when  its  physical 
basis  is  removed,  though  it  is  no  longer  perceptible  to  our 
senses.  It  is,  on  the  contrary,  quite  conceivable  theoreti- 
cally that  the  vital  forces  of  an  organism  should  suffer  no 
change  by  the  destruction  of  the  physical  bo'-is,  just  as  elec- 
tricity would  continue  to  subsist  in  a  world  composed  uni- 
versally of  non-conductors.  In  neither  case  could  the  force 
manifest  its  presence,  or  be  brought  into  any  perceptible 
relation  with  the  outer  world;  but  in  neither  case  should  we 
have  the  smallest  ground  for  assuming  that  tht  power  was 
necessarily  non- extant. 

b.  Organisation. — Having  decided  that  the  presence  of  a 
certain  physical  basis  or  peculiar  fonn  of  matter  is  essential  to 
the  manifestation  of  vital  phenomena,  we  may  next  puSS  on  to 
consider  •sN\i^\):itx  organisation^  or  the  presence  of  a  certain  de- 
finite structure,  is  one  of  the  essential  conditions  of  vitality. 
It  is  a  very  common  thing  to  speak  of  animals  as  if  they  were 
so  many  machines,  and  from  ont  limited  point  of  view  the 
comparison  is  a  fair  anJ  useful  one.  Every  machine,  however 
simple,  is  composed  of  <:ertain  definite  parts  which  have  cer- 
tain definite  relations  to  )ne  another ;  and  every  machine, 


1  i 


12  ELEMENTS   OK    V.lOl.or.Y. 

therefore,  has  what  in  the  case  of  an  animal  would  be  spoken 
of  as  "organisation."  Each  part  or  "organ"  of  the  machine 
has  certain  definite  functions,  and  the  machine  carries  out 
its  appointed  work,  when  supplied  with  the  necessary  force, 
in  virtue  of  the  harmonious  combination  and  interaction  of 
its  several  parts.  Most  animals,  in  the  same  way,  consist 
of  definite  parts  or  organs,  with  fixed  relations  to  one  ano- 
ther, and  each  discharging  its  own  work  or  function  in  the 
general  economy.  So  far  the  comparison  is  a  good  one, 
but  it  may  be,  and  has  been,  carried  too  far.  It  is  the  very 
essence  of  a  machine  that  it  should  consist  of  definite  parts. 
It  does  not  matter  whether  we  are  dealing  with  a  toasting- 
fork  or  a  oteam- engine,  we  have  in  all  cases  a  body  com- 
posed of  different  parts  performing  difterent  functions ;  and 
no  work  can  be  got  out  of  the  machine  unless  by  the  invo- 
cation of  a  separate  factor  to  supply  the  necessary  force. 
It  has  been  hastily  assumed  that  the  case  is  the  same  with 
animals,  and  the  common  simile  has  gone  far  to  foster  and 
diffuse  this  belief  It  has,  in  fact,  been  unhesitatingly  laid 
down  that  life  is  inseparably  connected  with  organisation  ; 
nay,  more,  it  has  even  been  asserted  that  life  is  the  resuU  of 
organisation.  The  falsity  of  this  belief,  however,  is  conclu- 
sively shown  by  the  study  of  the  minute  creatures  known  as 
i\iQ  Foraminifcra  {fig.  i).  These  little  animals  possess  the 
power  of  secreting  a  very  beautiful  and  elaborate  external 
envelope  or  shell,  and  they  thus  obtain  a  spurious  kind  of 
complexity  which  is  very  strikingly  at  variance  with  their 
real  simplicity.  In  point  of  fact,  the  bodies  of  the  Foram- 
inifera  exhibit  nothing  which  could  truly  be  termed  "  organ- 
isation." They  consist  simply  of  formless  and  structureless 
albuminous  matter.  They  are  not  composed  of  definite 
parts  or  organs,  and  they  are  in  no  proper  sense  to  be  com- 
pared to  machines.  Nevertheless,  they  live,  assimilate 
nourishment,  grow,  maintain  their  existence  against  hostile 
forces,  have  certain  relations  with  the  outer  world,  and 
reproduce  their  like.    The  highest  animal,  regarded  merely 


NATURE  AND  CONDITIONS  OF   LIFE. 


13 


as  an  animal,  can  do  no  more  than  this  ;  and  yet  the  Foram- 
inifera  attain  this  end  without  possessing  a  single  organ  of 


Fig.  T. — Forarr-.inlfera.  a  The  animal  o^ Nonionina,  after  the  shell  has  hecn  removed 
by  a  wer  k  acic  b  Grotnia  (after  Schultzc),  showing  the  shell  surrounded  by  a 
network  of  filaments  derived  from  the  body-subatance. 


any  ^ilnd.  These  minute  animalcules,  therefore,  show  in  an 
ex^emely  beautiful  and  instructive  manner,  that  organisa- 
ti  )n  is  only  a  result  of  life,  and  not  even  a  necessary  result. 
In  other  words,  we  learn  that  an  animal  is  organised,  or 
possesses  structure,  because  it  is  alive;  it  does  not  live 
because  it  is  organised. 

c.  Light. — In  one  sense  light  may  be  regarded  as  one  of 
2 


14  EI.F.MENTS   OF   BIOLOGY. 

the  essential  conditions  of  v'tality  ;  but  from  another  point 
of  view  it  is  wholly  unnecessary.  Light,  namely,  is  neces- 
"ary  for  animated  nature  as  a  whole,  but  is  by  no  means 
essential  to  all  living  beings'  regarded  as  individuals.  Many 
animals  spend  a  great  part  of  their  existence  in  total  dark- 
ness, and  some  pass  their  entire  life  without  access  to  the 
rays  of  the  sun.  Regarded,  however,  from  a  deeper  point 
of  view,  light  is  seen  to  be  absolutely  essential  to  life,  since 
vegetable  life  can  only  be  carried  on  under  the  influence  of 
sun-force.  All  animals,  as^we  shall  subsequently  see,  are 
dependent,  mediately  or  immediately,  upon  plants  '  their 
food ;  since  plants  alone  possess  the  power  of  bu  ig  up 
organic  compounds  out  of  inorganic  materials.  Plants, 
however,  can  perform  this  feat  of  vital  chemistry  only  when 
supplied  with  the  light-giving  and  cherriical  rays  cf  the  sun, 
so  that  light  is  an  absolute  prerequisite  for  life.  The  im- 
portance of  light  as  one  of  the  conditions  of  life,  will,  how- 
ever, be  spoken  of  at  greater  length  in  treating  of  the  food 
of  animals  and  plants,  and  the  distribution  of  animal  life  at 
great  depths  in  the  ocean. 

d.  Air. — The  presence  of  atmospheric  air,  or  rather  of 
free  oxygen,  appears  to  be  essential  to  animal  life,  and  a 
supply  of  oxygen  may  therefore  be  regarded  as  one  of  the 
extrinsic  conditions  of  vitality.  It  would  seem,  however, 
that  certain  low  vegetable  organisms  (vibriones  and  bacte- 
ria) flourish  in  an  atmosphere  of  carbonic  acid ;  so  that  free 
oxygen  cannot  be  looked  upon  as  being  an  indispensable 
requisite  of  life. 

e.  Temperature. — In  a  general  way,  the  higher  manifesta- 
tions of  life  are  only  possible  between  certain  limited  ranges 
of  temperature,  which  may  be  staged  as  varying  from  near 
the  freezing-point  to  120''  or  130°  Fahrenheit.  Some  of  the 
lower  forms  of  life,  however,  can  unquestionably  endure 
temperatures  much  more  extreme  ♦:han  these ;  ^nd  it  would 
appear  that  life  in  its  lowest  grades  is  not  impossible  at  tem- 
peratures considerably  below  the  freezing-point,  and  rising 


DEATH. 


15 


far  above  the  boiling-point  of  water  (from  20°  up  to  300'  F.) 
This  subject,  however,  will  be  treated  of  at  greater  length 
in  speaking  of  the  alleged  development  of  living  beings  de 
not'o  (Spontaneous  Generation). 

/.  Water. — Lastly,  it  may  be  remarked  that  no  vital  pro- 
cesses can  be  carried  on  except  in  the  presence  of  water. 
This,  however,  truly  depends  upon  the  fact  that  water  is  an 
essential  constituent  of  protoplasmic  or  albuminous  matter 
in  its  living  state.  The  necessity,  therefore,  for  a  "  physical 
basis  "  of  life,  carries  with  it  the  necessary  presence  of  water. 
Life,  however,  may  remain  in  a 
dormant  condition  during  long 
periods,  even  ih  the  total  ab- 
sence of  water.  ^S5r>l  2!^'5^J3'i£*:ISii^^ 

DEATH. 

The  non-fulfilment  of  any  of 
the  above-named  conditions  for 
any  length  of  time,  as  a  rule, 
causes  death,  or  the  cessation  of 
vitality  ;  but,  as  just  remarked, 
life  may  sometimes  remain  in  a 
dormant  or  "potential"  condi- 
tion for  an  apparently  indefinite 
length  of  time.  An  excellent 
illustration  of  this  is  afforded  by 
the  eggs  of  some  animals,  and 
the  seeds  of  many  plants ;  but 
a  more  striking  example  is  to 
be  found  in  the  Rotif'^ra  or 
"  Wheel-animalcules  "  (fig.  2). 

The  Rotifers  are  minute, 
mostly  microscopic  creatures, 
which  inhabit  almost  all  our 
ponds  and  streams.  Diminu- 
tive as  they  are,  they  are  nevertheless,  comparatively  speak- 


Fig.  2. — Rotifera.  Eosphora  auriin, 
one  of  the  Wheel-animalcules.  F^n- 
larged  about  250  Uiameters.  (After 
Gosse.) 


l6  ELEMENTS  OK  BIOLOGV. 

ing,  of  a  very  high  grade  of  organisation.  They  possesi^a 
mouth,  masticatory  organs,  a  stomacli,  and  alimentary  canal, 
a  distinct  and  well-develcped  nervous  system,  a  differen- 
tiated reproductive  apparatus,  and  even  organs  of  vision. 
Repeated  experiments,  however,  have  shown  the  remarkable 
fact,  that,  with  their  aquatic  habits  and  complex  organisa- 
tion, the  Rotifers  are  capable  of  submitting  to  an  apparently 
indefinite  deprivacion  of  the  necessary  conditions  of  their 
existence,  without  thereby  losing  their  vitality.  They  may 
be  dried  and  reduced  to  dust,  and  may  be  kept  in  this  state 
for  a  period  of  many  years ;  nevertheless,  the  addition  of  a 
little  water  will,  at  any  time,  restore  them  to  their  pristine 
vigour  and  activity.  It  follows,  therefore,  tnat  an  organism 
may  be  deprived  of  all  power  of  manifesting  any  of  the 
phenomena  which  constitute  what  we  call  life,  without  los- 
ing its  hold  upon  the  vital  forces  which  belong  to  it.  It 
seems,  however,  hardly  necessary  to  add  that  this  is  a  mere 
instance  of  revival  and  not  of  revitatisation.  The  desiccated 
Rotifers  are  not  truly  dead^  but  are  merely  in  ?,  state  of  sus- 
pended animation. 

USE   OF   THE   TERM   VITAL    FORCE. 

If,  in  conclusion,  it  be  asked  whether  the  term  "vital 
force  "  is  any  longer  permissible  in  the  mouth  of  a  scientific 
man,  the  question  must,  I  think,  be  answered  in  the  affirma- 
tive. Formerly,  no  doubt,  the  progress  of  science  was 
retarded  and  its  growth  checked  by  a  too  exclusive  reference 
of  natural  phenomena  to  a  so-called  vital  force.  Equally 
unquestionable  is  the  fact  that  the  development  of  Biological 
science  has  progressed  contemporaneously  with  the  succes- 
sive victories  gained  by  the  physicists  over  the  vitalists. 
Still,  no  physicist  has  hitherto  succeeded  in  explaining  any 
fundamental  vital  phenomenon  upon  purely  physical  and 
chemical  principles.  The  simplest  vital  phenomenon  has 
in  it  something  over  and  above  the  merely  chemical  and 
physical  forces  which  we  can  demonstrate  in  the  laboratory. 


s^ii 


USE  OF  THE  TERM  VITAL  FORCE. 


17 


It  is  easy,  for  example,  to  say  that  the  action  of  the  gastric 
juice  is  a  chemical  one,  and  doubtless  the  discovery  of  this 
fact  was  a  great  step  in  physiological  science.  Neverthe- 
less, in  spite  of  the  most  searching  investigations,  it  is  cer- 
tain that  digestion  presents  phenomena  which  are  as  yet 
inexplicable  upon  any  chemical  theory.  This  is  exemplified 
in  its  most  striking  form,  when  we  look  at  a  simple  organism 
like  the  Amoeba.  This  animalcule,  which  is  structurally 
little  more  than  a  mobile  lump  of  jelly,  digests  as  perfectly 
— as  far  as  the  result  to  itself  is  concerned — as  does  the 
most  highly  organised  animal  with  the  most  complex  diges- 
tive apparatus.  It  takes  food  into  its  interior,  it  digests  it 
without  the  presence  of  a  single  organ  for  the  purpose ;  and 
still  more,  it  possesses  that  inexplicable  selective  power  by 
which  it  assimilates  out  of  its  food  such  constituents  as  it 
needs,  whilst  it  rejects  the  remainder.  In  the  present  state 
of  our  knowledge,  therefore,  we  must  conclude  that  even  in 
the  process  of  digestion  as  exhibited  in  the  Amoeba  there  is 
something  that  is  not  merely  physical  or  chemical.  Simi- 
larly, any  organism  when  just  dead  consists  of  the  same 
protoplasm  as  before,  in  the  same  forms,  and  with  the  same 
arrangement ;  but  it  has  most  unquestionably  lost  a  some- 
thing by  which  all  its  properties  and  actions  were  modified, 
and  some  of  them  were  produced.  What  that  something  is 
we  do  not  know,  and  perhaps  never  shall  know ;  and  it  is 
possible,  though  highly  improbable,  that  future  discoveries 
may  demonstrate  that  it  is  merely  a  subtle  modification  of 
some  physical  force.  In  the  meanwhile,  as  all  vital  actions 
exhibit  this  mysterious  something,  it  would  appear  unphilo- 
sophical  to  ignore  its  existence  altogether,  and  the  term 
"  vital  force "  may  therefore  be  retained  with  advantage. 
In  using  this  term,  however,  it  must  not  be  forgotten  that 
we  are  simply  employing  a  convenient  expression  for  an 
unknown  quantity,  for  that  residual  portion  of  every  vital 
action  which  cannot  at  present  be  referred  to  the  operation 
of  any  known  physical  force. 


i8 


ELEMENTS   OF   BIOLOGY.     » 


!u 


It  must,  however,  also  be  borne  in  mind  that  this  residuum 
is  probably  not  to  be  ascribed  to  our  ignoranc  e,  but  that  it 
has  a  real  existence.  It  appears,  namely,  in  the  highest 
degree  probable  that  every  vital  action  has  in  it  something 
which  is  not  merely  physical  and  chemical,  but  which  is 
conditioned  by  an  unknown  force,  higher  in  its  nature  and 
distinct  in  kind  as  compared  with  all  other  forces.  The 
presence  of  this  "  vital  force  "  may  be  recognised  even  in  the 
simplest  phenomena  of  nutrition  ;  and  no  attempt  even  has 
hitherto  been  made  to  explain  the  phenomena  of  reproduc- 
tion by  the  working  of  any  known  physical  or  chemical 
force. 


.         t  ,          ,  i  -J  ; 

s  ; 

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■■.'..'  .'J    - '  •!»;•■,■••: 

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■.;  ,  s''^.  ;■'."  1-: 

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'nyj  \f<i  ■:i,y-4f 

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.    ■  .-i-     /.,  ■•■■,'■.•■•■■ 'K  :';';*  ti":      .'V'VU';  [ 
■  ^:';    '■■;■•::;:  ;i-;^ •■*>'"* ;\f.i?  ■ 


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t'..,  ■ 


CHAPTER    ir. 


^v«i.     :,'i 


DIFFERENCES   BETWEEN   ANIMALS   AND   PLANTS. 


Having  now  arrived  at  some  definite  notion  as  to  the  essen- 
tial characters  of  living  beings  in  general,  we  have  next  to 
consider  what  are  the  characteristics  of  the  U\o  great  divi- 
sions of  animated  nature.  What  are  the  characters  which 
induce  us  to  place  any  given  organism  in  either  the  animal 
or  vegetable  kingdom?  What,  in  short,  are  the  differences 
between  animals  and  i  slants  ? 

It  is  generally  admitted  that  all  bodies  which  exliibit  vital 
phenomena  are  capable  of  being  referred  to  one  of  the  two 
great  kingdoms  of  organic  nature.  At  the  same  time  it  is 
often  extremely  difficult  in  individual  cases  to  come  to  any 
decision  as  to  the  kingdom  to  which  a  given  organism  should 
be  referred,  and  in  many  cases  the  determination  is  purely 
arbitrary.  So  strongly,  in  fact,  has  this  difficulty  been  felt, 
that  some  observers  have  established  an  intermediate  king- 
dom, a  sort  of  no-man's-land,  for  the  reception  of  those 
debatable  organisms  which  cannot  be  definitely  and  posi- 
tively classed  either  amongst  vegetables  or  amongst  animals! 
Thus,  Dr  Ernst  Hseckel  has  proposed  to  form  an  intermediate 
kingdom,  which  he  calls  the  Regnum  Protisticum,  for  the 
reception  of  all  doubtful  organisms.  Even  such  a  cautious 
observer  as  Dr  Rolleston,  whilst  questioning  the  propriety 
of  this  step,  is  forced  to  conclude  that  "  there  are  organisms 


1 


Jj 


20  ELKMKNIS   OF   niOf^OOV. 

which  at  one  period  ol'  their  life  exhibit  an  aggregate  of 
phenomena  such  as  to  justify  us  in  speaking  of  them  as 
animals,  whilst  at  anotlier  they  appear  to  be  as  diiitinctly 
vegetable." 

In  the  case  of  the  higher  animals  and  plants  there  is  no 
difficulty;  the  former  being  at  once  distinguished  by  the 
possession  of  a  nervous  system,  of  motor  power  which  can 
be  voluntarily  exercised,  and  of  an  internal  cavity  fitted  for 
the  reception  and  digestion  of  solid  food.  The  higher 
plants,  on  the  other  hand,  possess  no  nervous  system  or 
organs  of  sense,  arc  incapable  of  independent  locomotion, 
and  are  not  provided  with  an  internal  digestive  cavity,  their 
food  being  wholly  fluid  or  gaseous.  These  distinctions, 
however,  do  not  hold  good  as  regards  the  lower  and  less 
highly  organised  members  of  the  two  kingdoms,  many  ani- 
njals  having  no  nervous  system  or  internal  digestive  cavity, 
whilst  many  plants  possess  the  power  of  locomotion ;  so 
that  we  are  compelled  to  institute  a  closer  comparison  in 
the  case  of  these  lower  forms  of  life. 

a.  Form. — As  regards  external  configuration,  of  all  char- 
acters the  most  ob\  ious,  it  must  be  admitted  that  no  abso- 
lute distinction  can  be  laid  down  between  plants  and  ani- 
mals. Many  of  our  ordinary  zoopLytes,  such  as  the  Hydroid 
Polypes,  the  sea-shrubs  and  corals — as,  indeed,  the  name  zoo- 
phyte implies — are  so  similar  in  external  appearance  to  plants 
that  they  were  long  described  as  such.  Amongst  the  Mol- 
luscoida,  the  common  sea-mat  (Flustra)  is  invariably  re- 
garded by  sea-side  visitors  as  a  sea-weed.  Many  of  the 
Protozoa  are  equally  like  some  of  the  lower  plants  (Proto- 
phyta) ;  and  even  at  the  present  day  there  are  not  wanting 
those  who  look  upon  the  sponges  as  belonging  to  the  vege- 
table kingdom.  On  the  other  hand,  the  embryonic  forms, 
or  "  zoospores,"  of  certain  undoubted  plants  (such  as  the 
Protococcus  nivalis,  Vaucheria,  &c.),  are  provided  with 
ciliated  processes  with  which  they  swim  about,  thus  coming 
so  closely  to  resemble  some  of  the  Infusorian  animalcules  as 


DIFFEKENCKS  HETWEEN  ANIMALS  AND  PLANTS.    21 

to  have  been  referred  to  that  division  of  the  Protozoa.  This 
is  also  the  case  with  some  adult  plants,  such  as  Voivox 
globaior  (fig.  3). 


Fig.  3. — Algic  and  Infusoria,  a  Ciliateil  zoospores  of  roMyJ-r?'/? ;  b  Ciliated  zoospore 
of  I'aitc/ieria;  c  Voivox  globator,  magnified;  d  Euplotes  Charon,  one  of  the 
lu/usoria,  magnified. 

b.  Internal  Structure. — Here,  again,  no  line  of  demarcation 
can  be  drawn  between  the  animal  and  vegetable  kingdoms. 
In  this  respect  all  plants  and  animals  are  fundamentally 
similar,  being  alike  composed  of  molecular,  cellular,  and 
fibrous  tissues. 

c.  Chemical  Composition. — Plants,, speaking  generally,  ex- 
hibit a  preponderance  of  ternary  compounds  of  carbon, 
hydrogen,  and  oxygen — such  as  starch,  cellulose,  and  sugar 
— ^>vhilst  nitrogen ised  compounds  enter  more  largely  into 
the  composition  of  animals.  Still  both  kingdoms  contain 
identical  or  representative  compounds,  though  there  may  be 
a  difference  in  the  proportion  of  these  to  one  another. 
Moreover,  the  most  characteristic  of  all  vegetable  com- 
pounds— viz.,  cellulose — has  been  detected  in  the  outer 
covering  of  the  sea-squirts  or  Ascidian  Molluscs ;  and  the 
so-called  "glycogen,"  which  is  secreted  by  the  liver  of  the 
Mammalia,  is  closely  allied  to,  if  not  absolutely  identical 
with,  the  hydrated  starch  of  plants.     As  a  general  rule,  how- 


22  ELEMENTS  OF   BIOLOGY. 

ever,  it  may  be  stated  that  the  presence  in  any  organism  of 
an  external  envelope  of  cellulose  raises  a  strong  presumption 
of  its  vegetable  nature.  In  the  face,  however,  of  the  facts 
above  stated,  the  presence  of  cellulose  cannot  be  looked  upon 
as  absolutely  conclusive.  Another  highly  characteristic  vege- 
table compound  is  chlorophyll^  the  green  colouring-matter  of 
plants.  Any  organism  which  exhibits  chlorophyll  in  any 
quantity,  as  a  proper  element  of  its  tissues,  is  most  pro- 
bably vegetable.  As  in  the  case  of  cellulose,  however,  the 
presence  of  chlorophyll  cannot  be  looked  upon  as  a  certain 
test,  since  it  occurs  normally  in  certain  undoubted  ani- 
mals [e.gt  Stentor,  amongst  the  Infusoria^  and  the  Hydra 
viridis^  or  the  green  Fresh-water  Polype,  amongst  the  Ccelen- 
tcrata). 

d.  Motor  Power. — This,  though  broadly  distinctive  of  ani- 
mals, can  by  no  means  be  said  to  be  characteristic  of  them. 
Thus,  many  animals  in  their  mature  condition  are  per- 
manently fixed,  or  attached  to  some  foreign  object ;  and  the 
embryos  of  many  plants,  together  with  not  a  few  adult 
forms,  are  endowed  with  locomotive  power  by  means  of 
those  vibratile,  hair-like  processes  which  are  called  "  cilia," 
and  which  are  so  characteristic  of  many  of  the  lower  forms 
of  animal  life.  Not  only  is  this  the  case,  but  large  num- 
bers of  the  lower  plants  such  as  the  Diatoms  and  Desmids, 
exhibit  throu'^hout  life  an  amount  and  kind  of  locomotive 
power  whir  aoes  not  admit  of  being  rigidly  separated  from 
the  movements  executed  by  animals,  though  the  closest  re- 
searches have  hitherto  failed  to  show  the  mechanism  where- 
by these  movements  are  brought  about. 

e.  Nature  of  the  Food. — Whilst  all  the  preceding  points 
have  failed  to  yield  a  means  of  invariably  separating  ani- 
mals from  plants,  a  distinction  which  holds  good  almost 
without  exception  is  to  be  found  in  the  nature  of  the  food 
taken  respectively  by  each,  and  in  the  results  of  the  conver- 
sion of  the  same.  The  unsatisfactory  feature,  however,  in 
this  distinction  is  this,  that  even  if  it  could  be  shown  to  be, 


DIFFERENCES  BETWEEN  ANIMALS  AND  PLANTS.    2j 


theoretically,  invariably  true,  it  would  nevertheless  be  prac- 
tically impossible  to  apply  it  to  the  greater  number  of  those 
minute  organisms  concerning  which  alone  there  can  be  any 
dispute. 

As  a  broad  rule,  all  plants  are  endowed  with  the  |)Ower  of 
converting  inorganic  into  organic  matter.  'yhe/oo(/o(  plants 
consists  of  the  inorganic  compounds,  carbonic  acid,  am- 
monia, and  water,  along  with  small  ciuantities  of  certain 
mineral  salts.  From  these,  and  from  these  only,  plants  arc 
capable  of  elaborating  the  proteinaceous  matter  or  protoplasm 
which  constitutes  the  physical  basis  of  life.  I'lants,  there- 
fore, take  as  food  very  simple  bodies,  and  manuflicturc  them 
into  much  more  complex  substances.  In  other  words,  by  a 
process  of  deoxidation  or  unburning^ rendered  possible  by 
the  influence  of  sunlight  only,  plants  convert  the  inorganic 
or  stable  elements — ammonia,  carbonic  acid,  Avater,  and 
certain  mineral  salts — into  the  organic  or  unstable  elements 
of  food.  The  whole  problem  of  nutrition  may  be  narrowed 
to  the  question  as  to  the  modes  and  laws  by  which  these 
stable  elements  are  raised  by  the  vital  chemistry  of  the 
plant  to  the  height  of  unstable  compounds.  To  this  gene- 
ral statement,  however,  an  exception  must  seemingly  be 
made  in  favour  of  certain  fungi,  which  require  organised 
compounds  for  their  nourishment. 

On  the  other  hand,  no  known  animal  jKJssesses  the  power 
of  converting  inorganic  compounds  into  organic  matter,  but 
all,  mediately  or  immediately,  are  dependent  in  this  respect 
upon  plants.  All  animals,  as  far  as  is  certainly  known, 
require  ready-made  proteinaceous  matter  for  the  mainten- 
ance of  existence,  and  this  they  can  only  obtain  in  the  first 
instance  from  plants.  Animals,  in  fact,  differ  from  plants 
in  requiring  as  food  complex  organic  bodies  which  they 
ultimately  reduce  to  very  much  simpler  inorganic  bodies. 
The  nutrition  of  animals  is  a  process  of  oxidation  or  burn- 
ing, and  consists  essentially  in  the  conversion  of  the  energy 
of  the  food  into  vital  work ;  this  conversion  being  effected 


24  ELEMENTS  OF   BIOLOGY. 

by  the  passage  of  the  food  into  living  tissue.  Plants,  there- 
fore, arc  the  great  manufacturers  in  nature,  animals  are  the 
great  consumers. 

Just,  however,  as  this  law  does  not  invariably  hold  good 
for  plants,  certain  fungi  being  in  this  respect  animals,  so  it 
is  not  impossible  that  a  limited  exception  to  the  universality 
of  the  law  will  be  foiyid  in  the  case  of  animals  also.  Thus, 
in  some  recent  investigations  into  the  fiiuna  of  the  sea  at 
great  depths,  a  singular  organism,  of  an  extremely  low  type, 
but  occupying  large  areas  of  the  sea-bottom,  has  been  dis- 
covered, to  which  Professor  Huxley  has  given  the  name  of 
Bathybius.  As  vegetable  life  is  extremely  scanty,  or  is 
altogether  wanting,  in  these  abysses  of  the  ocean,  it  has 
been  conjectured  th:^  this  organism  is  ])ossibly  endowed 
with  the  power — otherwise  exclusively  found  in  plants — of 
elaborating  organic  compounds  out  of  inorganic  materials, 
and  in  this  way  supplying  food  for  the  higher  animals  which 
surround  it.  The  water  of  the  ocean,  however,  at  these 
enormous  depths,  is  richly  charged  with  organic  matter  in 
solution,  and  this  conjecture  is  thereby  rendered  doubtful. 

Be  this  as  it  may,  there  remain  to  be  noticed  two  distinc- 
tions, broadly  though  not  universally  applicable,  which  are 
due  to  the  nature  of  the  food  required  respectively  by 
animals  and  plants.  In  the  first  place,  the  food  of  all 
plants  consists  partly  of  gaseous  matter  and  partly  of  matter 
held  in  Folution.  They  require,  therefore,  no  special  aper- 
ture for  its  admission,  and  no  internal  cavity  for  its  recep- 
tion. The  food  of  almost  all  animals  consists  of  solid 
particles,  and  they  are  therefore  usually  provided  with  a 
mouth  and  a  distinct  digestive  cavity.  Some  animals, 
however,  such  as  the  tape-worm  and  the  Gregarinae,  live 
entirely  by  the  imbibition  of  organic  fluids  through  the 
general  surface  of  the  body,  and  many  have  neither  a 
distinct  mouth  nor  stomach. 

Secondly,  plants  decompose  carbonic  acid,  retaining  the 
carbon  and  setting  free  the  oxygen,  certain  fungi  forming 


DIFFERENCES  BETWEEN  ANIMALS  AMD  PLANTS.    2$ 

an  exception  to  this  law.  The  reaction  of  plants  upon  the 
atmosphere  is  therefore  characterised  l>y  the  production  of 
free  oxygen.  Animals,  on  the  other  hand,  absorb  oxygen ' 
and  emit  carbonic  acid,  so  that  their  reaction  upon  the 
atmosphere  is  the  reverse  of  that  of  plants,  and  is  charac- 
terised by  the  production  of  carbonic  acid. 

Finally,  it  is  worthy  of  notice  that  it  is  in  their  lower  and 
not  in  their  higher  developments  that  the  two  kingdoms  of 
organic  nature  approach  one  another.  No  difficulty  is  ex- 
perienced in  separating  the  higher  animals  from  the  higher 
plants,  and  for  these  universal  laws  can  be  laid  down  to 
which  there  is  no  exception.  It  might,  not  unnaturally, 
have  been  thought  that  the  lowest  classes  of  animals  would 
exhibit  most  aflinity  to  the  highest  plants,  and  that  thus  a 
gradual  passage  between  the  two  kingdoms  v.ould  be  estab- 
lished. This  is  not  the  case,  however.  The  lower  animals 
are  not  allied  to  the  higher  plants,  but  to  the  lower ;  and  it 
is  in  the  very  lowest  members  of  the  vegetable  kingdom,  or 
in  the  embryonic  and  immature  forms  of  plants  little  higher 
in  the  scale,  that  we  find  such  a  decided  animal  gift  as  the 
power  of  independent  locomotion.  It  is  also  in  the  less 
highly  organised  and  less  specialised  forms  of  plants  that 
we  find  the  only  departures  from  the  great  laws  of  vegetable 
life,  the  deviation  being  in  the  direction  of  the  laws  of 
animal  life. 


(a;^:'\     ■^, 


■■•v,  r. 


f    r"    1-   !■■ 


,S  ■^■j  :  "■■; 


CHAPTER    IIT. 


DIFFERENCKS    DETWEEN    DIFFERENT   ORGANISMS. 


Morphology  and  Physiology. — The  next  point  which 
demands  notice  relates  to  the  nature  of  the  differences  by 
which  one  organism  may  be  separated  from  every  other,  and 
the  question  is  one  of  the  highest  importance.  Every  Hving 
being,  whether  animal  or  vegetable,  may  be  regarded  from 
two  totally  distinct,  and,  indeed,  often  apparently  opposite, 
points  of  view.  From  the  first  point  of  view  we  have  to 
look  solely  to  the  laws,  form,  and  arrangement  of  the  struc- 
tures of  the  organism;  in  short,  to  its  external  form  and 
internal  structure,  wholly  irrespective  of  the  manner  in 
which  it  discharges  its  vital  work.  This  constitutes  the 
science  of  Morphology  (Gr.  morphe,  form ;  logos,  a  discourse). 
From  the  second  point  of  view  we  have  to  study  the  vital 
actions  performed  L/  living  beings,  and  the  fufictions  dis- 
charged by  the  different  parts  of  the  organisn^,  separately 
or  collectively.  This  constitutes  the  science  of  Physiology. 
Morphology  not  only  treats  of  tlie  structure  of  living 
beings  in  their  fully-developed  condition  (Anatomy),  but  is 
also  concerned  with  the  changes  through  which  every  living 
being  has  to  pass  in  reaching  its  mature  or  adult  condition 
(Embryology  or  Development).  The  term  "  Histology,^' 
again,  is  further  employed  to  designate  that  branch  of  Mor- 
phology which  is  specially  occupied  with  the  investigation 


DIFFERENCES  BETWEEN  DIFFERENT  ORGANISMS.   2/ 


of  minute  or  mic.oscopical  tissues  (Gr.  histos,  a  web;  logos 
a  discourse).  '  •  •  ,, 

Physiology  treats  of  all  the  functions  exercised  by  living 
bodies,  or  by  the  various  definite  parts  or  organs  of  which 
most  living  beings  are  composed.  All  these  various  func- 
tions, however,  may  be  considered  under  three  heads : — 
I.  Functions  of  Nutrition^  divisible  into  functions  of  .'..bsorp- 
tion  and  Metamorphosis,  and  comprising  all  those  functions 
by  which  an  organism  is  enabled  to  live,  grow,  and  main- 
tain its  existence  as  an  individual,  2.  Functions  of  Repro- 
duction^ comprising  all  those  functions  whereby  fresh  indi- 
viduals are  produced  and  the  perpetuation  of  the  species  is 
secured.  3.  Functions  of  Relation  or  Correlation^  compris- 
ing all  those  functions  (such  as  sensation  and  voluntary 
motion)  whereby  the  outer  world  is  brought  into  relation 
with  the  organism,  and  the  organism  in  turn  is  enabled  to 
act  upon  the  outer  world. 

Of  these  three,  the  functions  of  nutrition  and  reproduc- 
tion are  often  spoken  of  collectively  as  the  "organic"  or 
"  vegetative  "  functions,  as  being  essential  to  bare  existence, 
and  as  being  common  to  animals  and  plants  alike.  The 
functions  of  relation,  again,  are  olcen  spoken  of  as  the 
"  animal "  functions  as  being  most  highly  developed  in 
animals.  These  functions,  however,  though  more  highly 
characteristic  of  animals,  are  not  peculiar  to  them,  but  are 
manifested  to  a  greater  or  less  extent  by  various  plants. 

All  tne  innumerable  differences  which  subsist  between 
different  organisms  may  be  classed  under  two  heads — 
morphological  and  physiological — corresponding  with  the 
two  aspects  of  every  living  being.  One  organism  differs 
from  another  either  morphologically^  in  the  fundamental 
points  of  its  structure  and  the  plan  upon  which  it  is  built, 
QX physiologically,  in  performing  a  different  amount  of  vital 
work,  in  a  different  manner,  or  with  different  instruments, 
or  both  morphologically  and  physiologically.  These  con- 
stitute the  only  modes  in  which  any  one  organism   can 


I    f 


28 


ELEMENTS  OF  BIOLOGY. 


^N 


differ  froi..  any  another;  and  they  may  be  considered  re- 
s^^^ctively  under  the  heads  of  "  Specialisation  of  Functions" 
and  "  Morphological  Type." 

a.  Specialisation  of  Functio?is. — All  animals  alike,  what- 
ever their  structure  may  be,  perform  the  three  great  phy- 
siological functions ;  that  is  to  say,  they  all  nourish  them- 
selves, reproduce  their  lil.e,  and  have  certain  relations  with 
the  external  world.  They  differ  from  one  another  physio- 
logically in  the  manner  in  which  these  functions  ire  per- 
formed. Indeed  it  is  only  in  the  functions  of  correlation 
that  it  is  possible  that  there  should  be  any  difference  in  the 
amount  or  perfection  of  the  function  performed  by  the 
organism,  since  nutrition  and  reproduction,  as  far  as  their 
results  are  concerned,  are  essentially  the  same  in  all  ani- 
mals. In  the  manner,  however,  in  which  the  same  results 
are  brought  about,  great  differences  are  observable  in  dif- 
ferent animals.  The  nutrition  of  such  a  simple  organism 
as  the  Amoeba  is,  indeed,  performed  perfectly,  as  far  as  the 
result  to  the  animal  itself  is  concerned — as  perfectly  as  in 
the  case  of  the  highest  animal — but  it  is  performed  with  the 
simplest  possible  apparatus.  It  may,  in  fact,  be  said  to  be 
performed  without  any  special  apparatus,  since  any  part  of 
thf;  surface  of  the  body  may  be  extemporised  into  a  mouth, 
and  there  is  no  differentiated  alimentary  cavity.  And  not 
only  is  the  nutritive  apparatus  of  the  simplest  character, 
but  the  function  itself  is  equally  simple,  and  is  entirely 
divested  of  those  complexities  and  separations  mto  second- 
ary functions  which  characterise  the  process  in  the  higher 
animals.  It  is  the  same,  too,  with  the  functions  of  repro- 
duction and  correlation  ;  but  this  point  will  be  more  clearly 
brought  out  if  we  examine  the  method  in  which  one  of  the 
three  primary  functions  is  performed  in  two  or  three 
examples.  Nutrition,  as  the  simplest  of  the  functions,  will 
best  answer  the  purpose. 

In  the  simpler  Protozoa^  such  as  the  Proteus  animalcule 
or  'Ama.'ba  (fig.  4),  it  can  hardly  be  said  that  there  are  any 


X. 


DIFFERENCES  BETWEEN  DIFFERENT  ORGANISMS.   29 

nutritive  organs* at  all,  at  any  rate  of  a  permanent  nature. 
The  prehension  of  food  is  effected  entirely  by  the  inter- 


r  V 


Fig.  4. — A,  Amoebae  developed  in  organic  infusions,  very  greatly  magnified 
(after  Bcale)  ;  ii,  Amaba  princeps  (after  Carter). 

vention  of  temporary  fingers  or  processes  of  the  body-sub- 
stance, which  can  be  thrust  out  at  will  from  any  point  of 
the  surface  of  the  body,  and  which,  when  retracted,  melt 
into  the  protoplasmic  body  without  leaving  a  trace  behind. 
There  is  no  mouth,  and  any  particle  of  food  seized  by  one 
of  these  temporary  arms  is  simply  engulfed  in  the  soft 
body,  as  one  might  thrust  a  stone  into  a  lump  of  dough. 
There  is  no  digestive  cavity,  and  there  are  no  digestive 
organs  of  any  kind.  Nevertheless  the  Amoeba  possesses  to 
the  full  the  power  of  assimilating  the  materials  which  it 
takes  as  food,  of  making  out  of  these  the  substances  which 
it  needs  for  its  growth  and  nourishment,  and  of  rejecting  all 
that  may  be  useless.  The  fluid  which  is  manufactured  out 
of  the  food,  and  which  may,  in  a  general  sense,  be  said  to 
correspond  to  the  blood  of  the  higher  animals,  is  probably 
propelled  to  all  parts  of  the  body  by  means  of  a  little  con- 
tractile bladder,  which  dilates  and  closes  at  regular  intervals. 
If  this  interpretation  of  the  facts  be  correct,  the  Anmba  is 
furnished  wTth  what  may  be  regarded  as  a  very  rudimentary 


30 


ET^KMENTS   OF   lilOLOGY. 


form  of  heart ;  but  it  is  not  quite  clear  that'  the  function  of 
this  little  hollow  sphere  is  as  above  stated.  Organs  by 
which  the  injurious  products  of  the  death  of  the  tissues  may 
be  eliminated,  are  absolutely  wanting ;  and  respiration,  if  it 
can  be  said  to  exist  at  all  as  a  distinct  function,  is  simply  . 
effected  by  the  general  su)  ce  of  the  body,  and  not  by  any  •? 
distinct  breathing  organ.  It  follows  from  this  that  not  only 
is  the  entire  process  of  nutrition  in  the  Amoeba  of  the  very 
simplest  character,  but  that  the  process  is  carried  out  with 
the  utmost  possible  absence  of  complication,  and  with  the 
very  simplest  machinery. 

In  a.  Coelenterate  animal,  such  as  one  of  the  sea-anem- '^ 
ones  (fig.  5),  the  function  of  nutrition  has  not  increased 
much  in  complexity,  but  the  means  for  its  performance  are 
somewhat   more   specialised.      A  distinct  and   permanent 


Fig.  S. — A,  Actinra  mesemhryanthemuin,  one  of  the  Sea-anemones  CafterJoVinston); 
B,  Section  of  the  same,  showing  the  mouth  {«),  the  stomach  {b),  and  the  body- 
cavity  (c). 


mouth  is  now  present,  and  this  is  surrounded  by  a  number  of 
prehensile  processes  or  "  tentacles,"  which  are  of  a  perma- 
nent nature,  and  are  not  produced  for  the  occasion  as  in  the 
case  of  the  temporary  arms  of  the  Amoeba,      The  mouth 


DIFFERENCES  BETWEEN  DIFFERENT  ORGANISMS.  3 1 

opens  into  a  permanent  digestive  cavity  or  stomach  ;  but 
this,  in  turn,  opens  directly  into  the  body-cavify  or  general 
chamber  enclosed  by  the  walls  of  the  body  (fig.  5,  B).  As 
a  result  of  this,  the  nutritive  fluid  prep:».red  from  the  food, 
which  we  may  call  the  blood,  gains  direct  access  to  the 
body-cavity,  where  it  is  largely  diluted  with  the  sea-water, 
which  is  also  freely  admitted  to  this  cavity.  The  nutritive 
fluid,  thus  weakened,  is  kept  in  constant  circulation  by 
means  of  innumerable  little  vibrating  hair-like  processes  or 
"cilia,"  with  which  the  lining  membrane  of  the  body-cavity 
is  furnished  ;  and  this  constitutes  the  only  representative  of 
the  circulatory  apparatus  of  the  higher  animals.  As  in  the 
Amoeba,  there  are  no  distinct  respiratory  organs,  and  no 
special  apparatus  by  which  effete  matters  may  be  got  rid  of. 


Fig.  6. — Diagrammatic  section  of  a  Whell;.  a  Mouth,  with  masticatory  apparatus  ; 
/' Salivary  glands  ;  <:  Stomach  ;  </(ir  Intestine,  surrounded  by  the  liver,  and  ter- 
minating in  the  anus  {e);  g  Gill ;  h  Heart ;  /Nervous  ganglion. 


In  a  Mollusc,  again,  such  as  the  Whelk  (fig.  6),  nutrition 


32 


ELEMENTS  OF  BIOLOGY. 


;,:■ 


>:/ 


hi 


is  a  much  more  complicated  process.  There  is  now  a  dis- 
tinct mouth  (a)  provided  with  a  masticatory  apparatus,  and 
opening  into  a  gullet  which  is  furnished  with  salivary  glands 
(d).  The  gullet  conducts  to  the  stomach,  which,  in  turn, 
opens  into  a  long  and  convoluted  intestine  {(i(f),  which  is 
completely  shut  off  from  the  general  cavity  of  the  body,  and 
which  terminates  in  a  permanent  aperture  (e),  by  which  the 
indigestible  portions  of  the  food  are  got  rid  of.  A  well- 
developed  liver  is  also  present.  The  nutritive  products  of 
digestion  are  now  propelled  through  all  parts  of  the  organ- 
ism by  a  permanent  contractile  organ  or  heart  (//).  Lastly, 
the  function  of  respiration  is  carried  on  by  distinct  and  com- 
plex organs  or  gills  (g),  whereby  the  blood  is  submitted  to 
the  action  of  the  oxygen  contained  in  the  surrounding 
water. 

It  is  not  necessary  here  to  follow  out  this  comparison 
further.  In  still  higher  animals  the  function  of  nutrition 
becomes  still  further  broken  up  into  secondary  functions, 
for  the  due  performance  of  which  special  organs  are  pro- 
vided, the  complexity  of  the  organism  thus  necessarily 
increasing  ^ari  passu  with  the  complexity  of  the  function. 
This  gradual  subdivision  and  elaboration  is  carried  out 
equally  with  the  other  two  physiological  functions — viz., 
reproduction  and  correlation — and  it  constitutes  what  is 
technically  called  the  "  specialisation  of  functions,"  though 
it  has  been  more  happily  termed  by  Milne-Edwards  "  the 
principle  of  the  physiological  division  of  labour."  As  has, 
however,  been  already  remarked,  in  any  physiological  com- 
parison of  organisms  one  with  another,  it  is  at  once  seen 
that  the  functions  of  relation  stand  in  quite  a  different  posi- 
tion to  that  occupied  by  the  functions  of  nutrition  and  re- 
production. As  far  as  these  last  are  concerned,  there  can 
be  no  difference  in  the  amount  or  perfection  of  the  function 
discharged  by  the  organism.  The  simplest  and  most 
degraded  of  animals — say  a  sponge — nourishes  itself  as 
perfectly,  as  far  as  the  result  to  itself  is  concerned,  as  does 


DIFI-EKKNCES  nETVVKKN  DIl'FKKENT  ORGANISMS.  ^^ 


the  highest  of  animals.  Nutrition  can  do  no  more  than 
maintain  the  body  of  any  animal  in  a  healthy  and  vigorous 
condition.  This  is  the  highest  possible  perfection  of  the 
function,  and  it  is  attained  as  fully  and  perfectly  by  the 
sponge  as  it  is  by  man  himself.  The  same  holds  good  of 
reproduction.  Whilst  the  functions  of  nutrition  and  repro- 
duction are  thus,  as  regards  their  essence  and  results,  the 
same  in  all  animals,  it  must  be  remembered  that  there  are 
enormous  differences  in  the  manner  in  which  the  functions 
are  discharged.  The  result  attained  is  in  all  cases  the 
same,  but  it  may  be  arrived  at  in  the  most  different  ways 
and  with  the  most  different  apparatus.  As  regards  the 
functions  of  relation,  on  the  other  hand,  we  have  every  pos- 
sible grade  of  perfection  exhibited  as  we  ascend  from  the 
lowest  members  of  the  animal  kingdom  to  the  highest.  So 
iiumerous,  in  fact,  are  the  changes  in  these  functions,  and 
so  great  the  additions  which  are  made  in  the  higher  organ- 
isms, that  it  may  be  doubted  if  there  exists  any  common 
element  by  which  a  comparison  can  be  drawn  on  this  head 
between  the  higher  and  lower  animals.  It  may  reasonably 
be  doubted  whether  in  this  respect  a  horse  or  a  dog  has 
anything  in  common  with  a  sponge. 

b.  Morphological  Type. — The  first  point  in  which  one  ani- 
mal may  differ  from  another  is  the  degree  to  which  the 
principle  of  the  physiological  division  of  labour  is  carried. 
The  second  point  in  which  one  animal  may  differ  from 
another  is  in  its  "  morphological  type ; "  that  is  to  say,  in 
the  fundamental  plan  upon  which  it  is  constructed.  By  one 
not  specially  acquainted  with  the  subject  it  might  be  readily 
imagined  that  each  species  or  kind  of  animal  was  con- 
structed upon  a  plan  peculiar  to  itself  and  not  shared  by 
any  other.  This,  however,  is  far  from  being  the  case  ;  and 
it  is  now  universally  recognised  that  all  the  varied  species  of 
animals — however  great  the  apparent  amount -of  diversity 
amongst  them — may  be  arranged  under  no  more  than  half- 
a-dozen  primary  morphological  types  or  plans  of  structure. 


34  ;■    .    ELEMENTS  OF  lUOLOGY.  -       - 

Upon  one  or  other  of  these  five  or  six  plans  every  known 
animal,  whether  living  or  extinct,  is  constructed.  It  follows 
from  the  limited  number  of  primitive  types  or  patterns,  that 
great  numbers  of  animals  must  agree  with  one  another  in 
their  morphological  type.  It  follows  also  that  all  so  agree- 
ing can  differ  from  one  another  only  in  the  sole  remaining 
element  of  the  question — namely,  by  the  amount  of  speciali- 
sation of  functions  which  they  exhibit.  Every  animal,  there- 
fore, as  Professor  Huxley  has  well  expressed  it,  is  the  resul- 
tant of  two  tendencies,  the  one  morphological,  the  other 
physiological. 

The  six  types  or  plans  of  structure,  upon  one  or  other  of 
which  all  known  animals  have  been  constructed,  are  techni- 
cally called  "sub-kingdoms,"  and  are  known  by  the  names 
Protozoa,  Coelenterata,  Annuloida,  Annulosa,  Mollusca,  and 
Vertebrata.  We  have,  then,  to  remember  that  every  mem- 
ber of  each  of  these  primary  divisions  of  the  animal  kingdom 
agrees  with  every  other  member  of  the  same  division  in 
being  formed  upon  a  certain  definite  plan  or  type  of  struc- 
ture, and  differs  from  every  other  simply  in  the  grade  of  its 
organisation ;  or,  in  other  words,  in  the  degree  to  which  it 
exhibits  specialisation  of  functions. 

It  is  to  be  remembered,  also,  that  whilst  all  naturalists 
recognise  distinct  plans  of  structure  or  "morphological 
types"  in  both  the  animal  and  vegetable  kingdoms,  all 
are  not  agreed  as  to  the  number  of  these  types.  In  other 
words,  all  zoologists  are  not  yet  agreed  as  to  the  characters 
which  should  be  regarded  as  constituting  a  distinct  "  mor- 
phological type."  The  result  of  this  is  that  different  authori- 
ties divide  the  animal  kingdom  into  a  different  number  of 
"sub-kingdoms."  Most  modern  naturalists,  however,  are 
agreed  as  to  the  morphological  distinctness  of  the  Protozoa^ 
Ccelenteraia,  Annulosa,  Mollusca,  and  Vertebrata.  There 
thus  remains  the  sub-kingdom  of  the  Annuloida  alone,  about 
which  any  serious  divergence  of  opinion  is  entertained. 
Following  Huxley,  this  division  is  here  regarded  as  having 


SUB-KINGDOM   1. — PROTOZOA. 


35 


the  rank  of  a  "sub-kingdom."  It  should  not  be  forgotten, 
however,  that  this  is  a  provisional  arrangement,  and  that 
future  researches  may  demonstrate  the  propriety  of  a  redis- 
tribution of  the  somewhat  heterogeneous  group  of  organisms 
at  present  included  under  this  head.  Subjoined  is  ?  brief 
synoptical  view  of  the  primary  divisions  of  the  animal  and 
vegetable  kingdoms,  with  the  characters  of  the  leading  groups 
comnrised  in  each. 


•  •  ANIMAL  KINGDOM. 

Sub-Kingdom  I. — Protozoa.  *  ^     : 

Animal,  simple  or  compound,  usually  very  minute.  Body  composed 
of  the  contractile,  struct jreless,  albuminpid  substance  termed  "sar- 
code  ; "  showing  no  composition  out  of  definite  segments ;  having  no 
nervous  system,  no  regular  circulatory  system,  no  definite  body-cavity, 
and  either  no  digestive  apparatus,  or  at  most  a  mouth  and  short  gullet. 
Reproduction  sexual  and  non-sexual  (fig.  7)« 


Protozoa.     A  Gregarine  ;  B  Rhizopod ;  C  Infusorian. 


Class  A.  Gregarinida. — Protozoa  which  live  parasitically  in  the 
interior  of  insects  and  other  animals,  which  are  destitute  of  a  mouth,  and 
have  no  power  of  throwing  out  prolongations  or  processes  of  the  body- 
substance  ("pseudopodia"). 

Class  B.  Rhizopoda  (Root-footed  Protozoa).— PrcJIbzoa  which  are 


36 


ELKMKNTS  UV   HIDLOGV. 


simple  or  lomjioimd,  and  have  the  power  of  lluowiiifj  out  and  retracting 
temjiorary  prolongations  of  the  body-substance  ("  pseudopodia  ").  A 
mouth  generally,  if  not  universally,  absent.     £x.  Sponges. 

Class  C.  Infusorfa  (Infusorian  Animalcules). — Protozoa  mostly 
with  a  mouth  and  short  gullet  ;  destitute  of  the  power  of  emitting 
jiseudopodia  ;  furnished  with  vibrating  hair-like  processes  (cilia)  or  con- 
tractile filaments  ;  the  body  composed  of  three  distinct  layers.  £x. — 
liell-animalcule.        ...  .,       ,         .       ., 


\i  1 


Sub-Kingdom  II.— Ccelentkrata.      • 

Animals  whose  alimentary  canal  communicates  freely  witli  the  general 
space  included  within  the  walls  of  the  body,  so  that  the  "body-cavity" 
comes  to  communicate  with  the  outer  medium  through  the  mouth. 
Body  composed  of  two  fundamental  layers  or  membranes,  an  outer 
layer  or  "  ectoderm,"  and  an  inner  layer  or  '•  endoderm."  No  central 
organ  of  the  circulation  or  distinct  blood-system;  in  inost  no  nervous 
system.  Skin  furnished  with  microscopic  stinging  organs  or  "thread- 
cells."  Reproductive  organs  in  all,  but  multiplication  often  by  non- 
sexual methods  (figs.  5  and  8).        ,  .        (        .'Vi 


A  B 

Fig,  8. — Ccclenterata.     A  l/yt/rn  r'u/^nris.  the  common  ficsh-wrtcr  Polype  (after 
Hincks).     I!  Diagrammatic  section  of  a  Hydra. 

Class  A.  liUbROZOA. — Walls  of  the  digestive  sac  not  separated  from 


SUn-KINGUOM  III.— ANNULOIDA 


57 


those  of  the  general  body-cavity,  the  two  coinciding  with  one  another. 
Reproductive  organs  external.  Ex.,  Fresh- water  I'olypes,  Sca-firs, 
Portuguese  Man-of-VVar,  Jelly-fishes,  Sea-hluhhers. 

Class  B.  Actinozoa — Stom.ich  opening  Iwlow  into  the  body-cavity, 
which  is  divided  into  a  number  of  compartments  by  vertical  partitions  or 
"mesenteries."  Reproductive  organs  internal.  Ex.,  Sea-.inemones, 
Star-corals,  Brain-corals,  Sea-pens,  Sea-shrubs,  Red-coral,  Vcnus's 
Girdle.  ,         > 


Sub-Kingdom  III.— Annuloida. 

Animals  in  which  the  alimentaiy  canal  (when  present)  is  completely 
shut  off  from  the  general  cavity  of  the  body,  and  in  which  there  is 
a  peculiar  system  of  canals,  distributed  through  the  body,  usually 
communicating  with  the  exterior,  and  termed  the  "water-vascular" 
system.  A  distinct  nervous  system,  and  sometimes  a  true  blood-vascu- 
lar system.  The  body  of  the  adult  never  composed  of  a  succession  of 
definite  rings  or  segments,  nor  provided  with  successive  pairs  of  append- 
ages disposed  symmetrically  on  the  two  sides  of  the  body.  Reproduc- 
tion rarely  asexual. 


Fig.  9. — Annuloida.     a  Holothuria  iuhulostt,  ctm  r^i  ihtSca-cncnmhen', 
b  and  c  Young  stages  of  the  same  (after  Jones). 

Class  A,  Echinodermata. — Integument  composed  of  numerous 
calcareous  plates  jointed  together,  or  leathery,  and  having  grains,  spines, 
or  tubercles  of  calcareous  matter  deposited  in  it.     Water- vascular  system 
3 


38 


ELEMENTS  OF  niOLOGY. 


generally  communicatinfj  with  the  exterior,  and  often  employed  in  loco- 
motion. Nervous  system  radiate.  Adult  generally  more  or  less  star- 
like or  "  radiate"  in  shape,  young  usually  showing  more  or  li-ss  distinct 
"bilateral  symmetry" — that  is,  showing  similar  parts  on  the  two  sides 
of  tlie  hody.  Ex.  Sea-urchins,  Star-fishes,  Itrittle-stars,  Sea-lilies, 
Sca-cucumhers. 

Class  U.  Scolecida.— Integument  soft,  and  destitute  of  calcareous 
matter.  Water-vascular  system  not  assisting  in  locomotion.  Nervous 
system  consisting  of  one  or  two  ganglia,  not  disposed  in  a  radiating 
manner,  liody  of  the  adult  sometimes  flattened,  sometimes  rounded 
and  wormiike.  Ex.  Tapeworms,  Flukes,  Hairworms,  Roundworms, 
Wheel-animalcules. 


Sub-Kingdom  IV. — Annulosa. 

Animal  composed  of  numerous  defmite  segments  or  "  somites,"  ar 
ranged  longitudinally  one  behind  the  other.  Nervous  system  consisting 
in  its  typical  form  of  a  double  chain  of  ganglia,  which  are  placed  along 
the  ventral  surface  of  the  body,  arc  united  by  longitudinal  cords,  and 
form  a  collar  round  the  gullet,  a  pair  of  ganglia  being  primitively  de- 
veloped in  each  segment.  Limbs  (when  y)resent)  disposed  in  pairs,  and 
turned  towards  that  side  of  the  body  on  which  the  main  masses  of  UiC 
nervous  system  are  situated  (fig.  lo). 


)^ysX>,-xx 


B 


Fig.  lo. — Annulosa.     A,  Diagram  of  Annulose  animal :  a  Digestive  tube,  b  Henit, 
c  Nervc-chaia.     B  Diagram  of  the  nervous  system  of  one  of  ttte  Annulota. 

Division  I.  Anarthropoda. — Locomotive  appendagei  (when  present) 
not  distinctly  jointed  or  articulated  to  the  body. 

Class  A.  Gephyrea. — Body  cylindrical,  not  definitely  segmented. 
Mouth  usually  with  a  circlet  of  tentacles.  Ventral  cord  of  the  nervous 
system  not  furnished  with  ganglia.    Ex.  Spoonworms. 

Class  B.  Annelida. — Body  cylindrical,  definitely  segmented.  A 
special  system  of  vessels  connected  with  respiration  ("pseudohaemal" 


I  i 


SUD-KINGDOM   V. — M0LLU3CA. 


39 


vessels).       A   ganKliatcd  ventral   ncrvc-chain.       Ex.  J^cches,   Earth- 
worms, Tubcworms,  Sandwortns, 

Ci^ss  ('.  CilyUTOGNATllA. — Head  furnished  with  rows  of  Imstlci. 
Nervoiiii  system  consisting;  of  a  cephalic  and  a  ventral  (ranglion  united 
by  cords  which  form  a  collar  round  the  gidlct.     Ex.  Sagitta. 

Division  II.  Artiiropoda. — Lo<omolive  appntda^es  jointed  or  arti- 
culated to  the  body. 

Class  1).  Crustacea. — Respiration  aquatic,  by  the  pencral  surface 
of  the  body  or  by  gills.  Two  pairs  of  antennne.  Locomotive  appen- 
dages more  than  four  pairs  in  number,  carried  upon  the  thorax,  and 
mostly  tlie  abdomen  also.  Ex.  Crabs,  Lobsters,  King-cral)s,  Wood- 
lice. 

Class  E.  Arachnida. — Respiration  aerial,  by  the  surface  of  the 
body,  by  pulmonary  chambers,  or  by  air-tubes  ("tracheae").  Antennae 
converted  into  jaws.  Head  and  thorax  amalgamated.  Four  pairs  of 
legs.  Abdomen  destitute  of  limbs.  Ex,  Spiders,  Scorjiions,  Mites, 
Ticks. 

Class  F.  MvRlAronA. — Respiration  aerial,  by  alr-tubcs  (tracheae) 
or  by  the  skin.  Head  distinct  ;  remainder  of  the  body  composed  of 
nearly  similar  segments.  Legs  more  than  eight  pairs  in  number,  and 
borne  partly  by  the  abdomen.  One  pair  of  antennae.  Ex.  Centipedes 
and  Millipedes. 

Class  G.  Insecta.  —  Respiration  aerial,  by  air-tubes  (tracheae). 
Head,  thorax,  and  abdomen  distinct.  One  pair  of  antennae.  Three 
pairs  of  legs  borne  on  the  thorax.  No  locomotive  limbs  on  the  segments 
of  the  abdomen.     Ex,  Beetles,  Flies,  Butterflies. 

Sub-Kingdom  V. — Mollusca. 

Animal  soft-bodied,  usually  with  a  hard  covering  or  shell.  Not 
exhibiting  distinct  segmentation.  Nervous  system  consisting  of  a  single 
gaiighon  or  of  scattered  pairs  of  ganglia.  A  distinct  heart  and  breath- 
ing organ,  or  neither  (fig.  ii). 

Division  L  Molluscoida. — A^enwis  system  consisting  of  a  single 
ganglion  or  a  principal  pair  of  ganglia.  No- heart,  or  an  imperfect 
one. 

Class  A.  Polyzoa. — Animal  always  forming  compound  growths  or 
colonies.  No  heart.  The  mouth  of  each  member  of  the  colony  sur- 
rounded by  a  circle  or  crescent  of  ciliated  tentacles.     Ex.  Sea-mat. 

Class  B.  Tunicata. — Animal  simple  or  compound,  enclosed  in  a 
leathery  or  gristly  case.     An  imperfect  heart.     Ex.  Sea-squirt. 

Class  C.  Brachiopoda.  —  Animal  simple,  enclosed  in  a  bivalve 
shell  Mouth  furnished  with  two  long  fringed  processes  or  "arms." 
^jr.  Lamp-shells.  ..  ...... 


I 


t . 


40 


KI.EMENTS  OF  BIOLOGY. 


Division  II.  Mollusc/.  PRorRR. — Nervous  system  consisting  of  three 
principal  pairs  of  ganglia.  Heart  well  developed,  of  at  least  tuv 
(hainbas. 


,--^ 


Fig.  II. — Mollusca.     Diagram  of  a  Cuttle-fish.     (Altered  from  Huxley.) 

Class  D.  Lamellibranchiata. — No  distinct  head  or  teeth.  Eody 
enclosed  in  a  bivalve  shell.  One  or  two  leaf-like  gills  on  each  side  of 
the  body.    Ex.  Oyster,  Mussel,  Cockle. 

Class  E.  Gasteropoda. — A  distinct  head  and  toothed  tongue. 
Shell,  when  present,  univalve  or  multivalve,  never  bivalve.  Locomo- 
tion effected  by  creeping  about  on  the  flattened  under-surface  of  the 
body  ("  foot"),  or  by  swimming  by  means  of  a  fin-like  modification  of 
the  same.     Ex.  Whelk,  Periwinkle,  Snail. 

Class   F.    Pteropoda. — Animal  oceanic,   swimming  by  means  of 
two  wing-like  appendages,  one  on  each  side  of  the  head.     Size  minute. 
Ex.  Cleodora.  '    v      • 

Class-  C-.  Cephalopoda. — Animal  with  eight  or  more  processes  or 
•'arm'-  '  placed  round  the  mouth.      Mouth  armed  with  jaws  and  a 


I  :li 


SUB-KINGDOM  VI. — VERTEBRATA. 


41 


toothed  tongue.  Two  or  four  plume-like  gills.  In  front  of  the  body  a 
muscular  tube  ("funnel"),  through  which  is  expelled  the  water  which 
has  been  used  in  respiration.  An  external  shell  in  some,  an  internal 
skeleton  in  others.     Ex.  Cuttle-fishes,  Nautilus. 


Sub-Kingdom  VI.— Vertebrata. 

Body  composed  of  a  number  of  definite  segments  placed  one  behind 
the  other  in  a  longitudinal  series.  The  main  masses  of  the  nervous 
system  are  placed  upon  the  dorsal  aspect  of  the  body,  and  are  shut  off 
from  the  general  body-cavity.  The  limbs  (when  present)  are  turned 
away  from  that  side  of  the  body  on  which  the  main  masses  of  the 
nervous  system  are  placed,  and  are  never  more  than  four  in  number. 
In  most  cases  i.  backbone  or  "vertebral  column"  is  present  in  the 
fully-grown  animal.     (Fig.  12.) 


Fig.  12.— Vertebrata.     Skeleton  of  the  common  Perch  (/'^w<j^Kt'/a///«). 

Class  A.  Pisces  (Fishes). — Breathing  organs  in  the  form  of  gills  ; 
heart,  when  present,  usually  of  two  chambers,  rarely  of  three ;  blood 
cold  ;  limbs,  when  present,  converted  into  fins. 

Class  B.  Amphibia  (Amphibians), — Breathing  organs  of  the  young, 
gills ;  of  the  adult,  lungs,  either  alone  or  associated  with  gills.  Heart  of 
the  young  of  two  chambers,  of  the  adult  of  three  chambers.  Blood 
cold.  Skull  jointed  to  the  backbone  by  two  articulating  surfaces 
("  condyles  ").     Limbs  never  converted  into  fins. 

Class  C.  Reptilia  (Reptiles). — Breathing  organs  in  the  form  of 
lungs,  never  in  the  form  of  gills.     Heart  three-chambered,  rarely  four- 
chambered,  the  pulmonary  and  systemic  circulations  connected  together, 
either  in  the  heart  or  in  its  immediate  neighbourhood.     Blood  cold. 
Skull  jointed  to  the  backbone  by  a  single  articulating  surface  or  condyle. 


42 


ELEMENTS   OF  BIOLOGY. 


Each  half  of  the  lower  jaw  composed  of  several  pieces.  Appendages  of 
the  skin  in  the  form  of  homy  scales  or  bony  plates. 

Class  D.  Aves  (Birds).— Respiratory  organs  in  the  form  of  lungs. 
Lungs  connected  with  air-receptacles  placed  in  various  parts  of  the 
body.  Heart  four-chambered.  Blood  warm.  Skull  connected  with 
the  backbone  by  a  single  articulating  surface  or  "condyle."  Each  half 
of  the  lower  jaw  composed  of  several  pieces.  Appendages  of  the  skin 
in  the  form  of  feathers.  Fore-limbs  converted  into  wings.  Animal 
oviparous. 

Class  E.  Mammalia  (Quadnipeds). — Breathing  organs  in  the  form 
of  lungs,  which  are  never  connected  with  air-receptacles  placed  in 
different  parts  of  the  body.  Heart  four-chambered.  Blood  warm. 
Skull  connected  with  the  backbone  by  two  articulating  surfaces  or 
"  condyles."  Each  half  of  th*^  lower  jaw  composed  of  a  single  piece. 
Appendages  of  the  skin  in  the  form  of  hairs.  Young  nourished  by 
means  of  a  special  fluid — the  milk — secreted  by  special  glands — the 
mammary  glands.     Animal  viviparous. 

VEGETABLE  KINGDOM. 


1  j| 


# 


I'M 


m 

i;  ' 


isai  k 


Sun-KiNGDOM  I.— Cryptogams. 

Plants  destitute  of  tru'^  flowers  with  stamens  and  pistils.  No  tru« 
seeds,  but  simple  cellules  or  "  spores,"  in  which  there  is  no  embryo 
prior  to  germination. 

Class  I.  Thallopiiyta. — Stem  and  foliage  undistinguishable,  com- 
posed of  cellular  tissue  only.     £x.  Lichens,  Algse,  and  Fungi. 

Class  II.  Anophyta. — Stem  and  foliage  distinct  or  confluent,  of 
cellular  tissue  only.     £x.  Mosses  and  Liverworts. 

Class  III.  AcROOENiE. — Stem  with  woody  tissue  and  vessels,  giowing 
at  its  summit,  and  usually  with  distinct  foliage.  JSx.  Horse-tails,  Club- 
mosses,  Ferns. 

Sub-Kingdom  II. — Phanerogams. 

Plants  producing  true  flowers  with  stamens  and  pistils.  True  seeds 
containing  an  embryo. 

Section  A.  Monocotyledones.— .SWj  wHA  one  cotyledon  or  seed- 
leaf.  Stems  ^*  endogenous  "  tuith  no  manifest  distinction  into  bark,  woody 
and  pith. 

Class  I.  Endogens. — Leaves  parallel-veined,  permanent.  Root 
like  the  stem  internally.     Ex.  Palms,  Lilies,  Grasses. 

Class   II.    Dictyogens. — Leaves  net-veined,   deciduous.      Root 
with  the  wood  in  a  solid  concentric  circle.     Ex.  Sarsaparilla. 


■It;: 


SUB-KINGDOM  11. — PIIANEROGAM^E. 


43 


Section  B.  Dicotyledones. — Seeds  with  two  or  Ptore  carpels.  Stem 
"  exogetwus"  with  bark,  wood,  and  pith.     Leaves  netted-veined. 

Class  III.  GYMNOSPERMi«.— Seeds  naked,  the  pollen  acting  directly 
upon  their  surface.     Ex.  Pines  and  Cycads. 

Class  IV.  Angiosperm^.  —  Seeds  enclosed  in  seed-vessels,  the 
pollen  acting  through  their  tissues.  Ex.  Oak,  Beech,  and  most  ordinary 
trees  and  shrubs. 


,'(' .  "*  )f  ••'"  •^' 


"Vr' ;  •■  I : 


■,v„',!r.-j.,:  I,  „,/:,','; 


CHAPTER    IV. 


ANALOGY,    HOMOLOGY,    HOMOMORPHISM,    MIMICRY,    AND 
CORRELATION    OF   GROWTH. 

I.  Analogy.  —  The  term  "analogue"  was  defined  by 
Owen  to  be  "  a  part  or  organ  in  one  animal  which  has  the 
same  functions  as  another  part  or  organ  in  a  different  ani- 
mal." In  other  words,  those  parts  or  organs  are  analogous 
which  resemble  one  another  physiologically  and  discharge 
the  same  functions^  wholly  irrespective  of  what  their  funda- 
mental structure  may  be.  In  most  cases  the  organs  which 
would  ordinarily  be  called  "  analogous  "  are  such  as  differ 
from  one  another  in  structure,  at  the  same  time  that  they 
discharge  the  same  duties.  Thus  the  wings  of  a  bird  and 
the  wings  of  an  insect  are  analogous  organs,  since  they  are 
both  organs  of  flight,  and  serve  to  sustain  their  possessor  in 
the  air.  They  are,  however,  in  no  way  similar  to  one  an- 
other except  when  regarded  from  this  physiological  point  of 
view  j  and  they  differ  altogether  from  a  morphological  aspect, 
being  in  no  way  formed  on  the  same  fundamental  plan.  It 
often  happens,  however,  that  "analogous"  organs  have  the 
deeper  relation  to  one  another  of  being  constructed  upon 
the  same  morphological  plan,  in  which  case,  in  addition  to 
their  analogy,  we  have  to  consider  the  relationship  which  is 
known  by  the  general  name  of  "  homology." 

II.  Homology. — According  to  Owen,  a  "homologue"  is 
"  the  same  organ  in  different  animals  under  every  variety  of 


HOMOLOGY. 


45 


form  and  function."  In  other  words,  those  organs  or  parts 
in  different  animals  are  homologous^  which  agree  with  one 
another  morphologically  in  their  fundamental  structure^  quite 
irrespective  of  what  functions  they  discharge  in  the  economy. 
Thus  the  arm  of  man,  the  fore-leg  of  the  dog,  and  the  wing 
of  a  bird,  are  constructed  upon  the  same  morphological 
type,  and  are  therefore  homologous  (fig.  13).  They  are 
not,  however,  analogous,  since  they  perform  wholly  different 
functions,  the  first  being  an  organ  of  prehension,  the  second 
devoted  to  terrestrial  progression,  and  the  third  an  organ  of 
flight.     There  are,  however,  many  cases  in  which  organs  in 


r  .. 


ABC 

Fig.  13. — A  Arm  of  Man  ;  B  Fore-leg  of  Dog  ;  C  Wing  of  Bird  :  A  Humerus; 
r  Radius ;  m  Ulna ;  c  Carpus ;  ttic  Metacarpus ;  /  Phalanges. 

different  animals  are  not  only  constructed  of  the  same 
essential  parts,  but  also  discharge  the  same  functions,  thus 
coming  to  be  both  homologous  and  analogous. 

Besides  the  homologies  which  subsist  between  organs  in 
different  animals,  there  are  two  kinds  of  homology  which 
may  be  present  in  the  different  parts  of  the  same  animal, 


MM 


46 


ELEMENTS  OF  BIOLOGY. 


and  which  are  known  as  "  serial  homology "  and  "  lateral 
homology." 

Serial  homology  is  established  by  the  presence  in  a  single 
animal  of  a  succession  of  two  or  more  parts  which  are 
placed  in  a  longitudinal  series  one  behind  the  other,  and 
which  have  the  same  fundamental  structure.  In  no  animals 
is  this  phenomenon  better  seen  than  in  the  Annulosa,  such 
as  the  great  majority  of  the  Crustaceans,  in  which  it  is  easy 
to  see  that  the  body  is  composed  of  a  longitudinal  succes- 
sion of  rings  or  segments,  placed  in  a  row  one  behind  the 
other,  and  essentially  alike  in  their  structure  (fig.  14).     In 


Fig.  14. — Fairy  Sluiinp  {ChiroceJ>halns  diapluinud).     After  Baird. 

the  majority  of  cases,  however,  whilst  these  serial  parts  have 
a  fundamentally  identical  structure,  and  are  clearly  built 
upon  a  common  plan,  they  are  not  all  alike ;  but  they  are 
modified  in  different  regions  of  the  body  to  fit  them  for  the 
fulfilment  of  special  functions.  Certain  of  the  segments, 
therefore,  differ  physiologically  from  certain  others,  and  thus 
come  to  differ  morphologically  as  well.  There  are  other 
cases,  however,  as  the  Centipedes  (fig.  15),  for  instance,  in 


Fig.  ig. — (  cntipede  (5'ft7/<j/^«^w).     After  Jones. 

which  the  greater  number  of  the  serial  parts  are  exactly 
similar  both  in  structure  and  in  function ;  and  these,  per- 


HOMOLOGY, 


4; 


it . 


haps,  may  be  more  properly  regarded  as  examples  of  "  vege- 
tative repetition  "  of  parts  than  as  being  instances  of  true 
serial  homology.  This  is,  at  any  rate,  certainly  the  case 
with  the  flattened  segments  which  make  up  the  great  bulk 
of  what  is  ordinarily  called  a  "  tapeworm,"  and  which  are 
produced  as  genuine  buds  from  a  rounded  "  head,"  which 
they  in  no  way  resemble  either  in  structure  or  in  function. 

In  Vertebrate  animals  serial  homology  is  a  much  less 
evident  phenomenon  than  in  the  cases  we  have  been  con- 


r  - 


/ 


Fig.  16. — Fore-limb  of  Chimpanzee  (after 
Owen).  —  A  Humerus;  r  Radius;  m 
Ulna ;  d  Bones  of  the  wrist,  or  carpus ; 
*H  Metacarpus;  /  Phalanges  of  the 
fuigers. 


Fig.  17. — Hind-limb  of  the  Chimpanzee 
(after  Owen).— /  Femur;  /Tibia;  * 
Fibula  ;  r  Bones  of  the  ankle,  or  tar- 
sus ;  m  Metatarsus;  /  I'halanges  of 
the  toes. 


sidering,  but  it  nevertheless  exists  in  a  well-marked  form. 
Thus  the  vertebral  column  or  backbone  is  composed  of  a 


4-8 


ELEMENTS  OF  BIOLOGY. 


longitudinal  succession  of  bony  segments  which  are  formed 
upon  a  common  structural  plan,  and  exhibit  essentially  the 
same  parts,  though  modified  in  different  regions  of  the 
spine.  Much  more  conspicuous,  however,  than  the  serial 
homology  of  the  segments  of  the  spine,  is  the  homology 
presented  by  the  fore  and  hind  limbs  of  the  Vertebrata  (figs. 
1 6,  17).  These,  in  all  instances,  can  be  shown  to  be  modi- 
fications of  a  common  plan — that  is  to  say,  they  consist  of 
parts  which  are  fundamentally  similar  to  one  another, 
though  very  often  the  limbs  may  discharge  different  func- 
tions, and  may  thus  come  to  differ  considerably  in  structure. 
Lateral  homology  consists  in  the  structural  identity  of  the 
parts  on  the  two  sides  of  the  body  in  any  given  animal. 
When  this  identity  is  complete,  the  animal  becomes  "bi- 
laterally symmetrical ; "  or,  in  other  words,  exhibits  similar 
and  symmetrical  parts  on  the  two  sides  of  the  body.  Some 
animals,  however,  never  exhibit  any  lateral  homology  or 
bilateral  symmetry  at  any  period  of  their  lives ;  and  others 
only  exhibit  it  when  young,  and  lose  it  more  or  less  coipi- 
pletely  when  adult.  It  has  been  endeavoured  to  show  that 
lateral  homology  is  the  result  of  the  similar  way  in  which 
conditions  affect  the  right  and  left  sides  of  the  body  respec- 
tively (Herbert  Spencer) ;  but  this  does  not  appear  to  be  in 
any  way  an  adequate  explanation.  In  the  first  place,  there 
are  many  animals  which  exhibit  bilateral  symmetry  in  their 
superficial  structures  and  appendages,  but  which  show  no 
such  symmetry  in  the  immediately  contiguous  internal 
organs ;  though  it  can  hardly  be  pretended  that  the  effect 
of  similar  conditions  extends,  say,  an  inch  below  the  sur- 
face, but  stops  short  at  that  point.  In  the  second  place, 
there  are  many  animals  which  belong  to  types  in  which 
bilateral  symmetry  is  the  rule,  but  which,  nevertheless, 
normally  and  regularly  exhibit  a  want  of  symmetry,  either 
in  their  appendages  or  in  their  internal  organs.  Thus  it 
cannot  be  pretended  that  the  conditions  which  affect  one 
side  of  a  Lobster  are  different  to  those  which  act  on  the 


HOMOLOGY. 


49 


other,  and  yet  the  nipping-claw  on  one  side  is  always  bigger 
than,  and  differently  shaped  to,  the  nipping-claw  on  the 
other.  Again,  otiier  Crustaceans  have  certain  appendages 
on  one  side  which  do  not  exist  at  all  upon  the  other  side. 
Many  animals,  again,  have  the  internal  organs  of  one  side 
of  the  body  either  quite  rudimentary  or  completely  atro- 
phied— as  occurs,  for  example,  in  the  Snakes — and  yet  they 
show  no  external  indication  of  this  want  of  symmetry,  nor 
can  we  assert  that  the  two  sides  of  the  body  have  been  ex- 
posed to  conditions  in  any  way  dissimilar.  ],astly,  it  can- 
not be  shown  that  those  animalL  which  exhibit  no  lateral 
symmetry,  or  in  which  this  symmetry  is  masked,  have  been 
exposed  to  conditions  in  any  respect  different  to  those 
affecting  the  bilaterally  symmetrical  animals  which  accom- 
pany them ;  nor  can  it  be  shown  that  such  animals  have 
been  exposed  to  one  set  of  conditions  on  one  side  of  their 
body,  and  another  set  of  conditions  on  the  other  side. 

Homogeny  and  Homoplasy. — To  meet  the  requirements  of 
those  who  hold  the  doctrine  of  the  "  evolution  "  of  all  exist- 
ing species  of  organisms  from  other  different  pre-existent 
forms,  Mr  Ray  Lankester  has  recently  proposed  to  super- 
sede the  term  "  homology,"  and  to  substitute  for  it  the  two 
tenns  "  homogeny  "  and  "  homoplasy."  On  this  view  only 
those  organs  in  different  animals  are  "  homogenous  "  which 
owe  their  resemblances  to  genetic  community  of  origin ;  or, 
in  other  words,  to  their  having  had  "  a  si»gle  representative 
in  a  common  ancestor."  On  the  other  hand,  Mr  Lankester 
asserts  that  when  "  identical  or  nearly  similar  forces,  or 
environments,  act  on  two  or  more  parts  of  an  organism 
which  are  nearly  or  exactly  alike,*  the  resulting  modifications 
of  the  various  parts  will  be  exactly  or  nearly  alike ; "  and 
further,  that  "  if,  instead  of  similar  parts  in  the  same  organ- 
ism, we  suppose  the  same  forces  to  act  on  parts  in  two 
organisms,  which  parts  are  exactly  or  nearly  alike*  and 
sometimes  homogenetic,  the  resulting  correspondences  called 
-.^.,    ,,  ,,f  •  The  italics  are  the  author's,  net  Mr  Lankester's. 


50  ELEMENTS  OF  BIOLOGY. 

forth  in  the  several  parts  of  the  two  organisms  will  be  nearly 
or  exactly  alike."  For  agreements  produced  in  this  v^rxy  the 
term  "  homoplasy  "  is  proposed. 

Two  very  strong  objections  seem  to  render  the  acceptance 
of  those  terms  inadmissible.  As  regards  the  term  "  homo- 
geny,"  as  above  defined,  it  is  to  be  remarked  that  its  value 
depends  wholly  and  solely  upon  the  value  which  may  be 
attached  to  the  hypothesis  of  "  evolution."  Many  authori- 
ties of  no  small  weight  do  not  accept  this  hypothesis,  and 
to  such  the  tenn  '*  homogeny  "  is  worse  than  useless,  for  it 
implies  a  relationship  between  "  homologous "  organs,  in 
which  they  do  not  believe.  For  general  use,  therefore,  we 
must  prefer  the  term  "  homologous  "  to  that  of  "homogene- 
tic" In  the  second  place,  as  regards  the  term  **  homo- 
plasy," a  reference  to  the  above  definition  will  show  that  it 
is  proposed  for  those  resemblances  which  are  produced  in 
parts  or  organs  of  the  same  animal,  or  of  different  animals, 
by  identical  forces  or  environments,  the  said  parts  being 
nearly  or  exactly  alike  to  begin  with.  If,  however,  the 
"homoplastic"  parts  are  primarily  alike  before  they  begin 
to  be  acted  upon  by  similar  forces,  then  they  would  seem  to 
be  "homogenetic ;"  and  no  fresh  term  is  required  to  indi- 
cate the  fact  that  similar  conditions  acting  upon  parts  sub- 
stantially the  same  will  produce  similar  results.  No  attempt 
is  made  to  explain  how  the  parts  in  question  come  to  be 
"  nearly  or  exactly  alike "  in  the  first  instance;  and,  in  the 
absence  of  such  an  explanation,  it  seems  clear  that  it  is  a 
mere  assumption  that  the  likeness  which  we  at  present 
observe  between  them  is  only  the  result  of  the  action  of 
"  identical  or  nearly  similar  forces." 

III.  HoMOMORPHiSM. — Many  examples  are  known,  both 
in  the  animal  and  the  vegetable  kingdom,  in  which  families 
widely  removed  from  one  another  in  their  fundamental 
structure,  nevertheless  present  a  singular  and  sometimes 
extremely  close  resemblance.  For  this  phenomenon  the 
term  "  homomorphism  "  has  been  proposed,  and  such  forms 


HOMOMORPHISM.  51 

are  said  to  be  "  homomorphic."  Thus,  the  Hydroid  Zoo- 
phytes and  the  Sea-mosses  {Polyzod)  arc  singularly  like  one 
another  in  external  form  ;  so  much  so  that  they  have  often 
been  classed  together,  whereas  they  differ  very  greatly  in 
their  anatomical  characters.  Many  other  instances  njight 
be  adduced  of  this  close  external  resemblance  between 
animals  and  plants  which  have  little  or  no  real  relationship 
with  one  another  ;  and  in  many  cases  these  "  representative 
forms  "  appear  to  be  able  to  fill  each  other's  places  in  the 
general  economy  of  nature.  Tliis  is  so  far  tme,  at  any  rate, 
that  "  homomorphous  "  forms  are  generally  found  in  differ- 
ent parts  of  the.  earth's  surface.  Thus,  the  place  of  the 
Cacti  in  South  America  is  taken  by  \\\q.  Euphorbia  of  Africa; 
or,  to  take  a  zoological  illustration,  many  of  the  different 
orders  of  the  mammals  are  rtprcsmtcd  by  the  sections  of  the 
single  order  of  the  Marsupialia.  This  order,  namely,  is  the 
almost  exclusive  possessor  of  the  entire  Continent  of  Aus- 
tralia, and  being  thus  confronted  with  very  varying  condi- 
tions, and  enjoying  the  almost  unlimited  freedom  of  an 
enormous  area,  the  order  has  to  singly  discharge  the  func- 
tions which  are  elsewhere  performed  by  several  orders. 
Homomorphous  forms,  however,  are  not  universally  found 
in  areas  widely  removed  from  one  another ;  and  it  is  very 
difficult  to  account  for  their  origin  in  any  case.  The  older 
view,  advocated  by  the  late  Edward  Forbes,  was  that  "  re- 
presentative "  forms,  similar  to,  but  specially  distinct  from, 
one  another,  were  created  independendy  in  areas  which 
presented  similar  conditions  and  environments.  The  more 
modem  view  would  regard  "homorphous"  forms  as  pro- 
duced by  the  action  of  similar  conditions  upon  organisms 
primitively  not  very  unlike  one  another;  so  that  "homo- 
morphism  "  would  thus  become  a  form  of  the  "homoplasy" 
of  Mr  Lankester.  This  explanation,  however,  would  still 
leave  much  of  this  subject  unexplained. 

IV.    Mimicry.  —  Many  instances  are  known  amongst 
animals  in  which  certain  species  put  on  the  external  charac- 


i 


52 


ELEMENTS  OF  BIOLOGY. 


ters  of  other  species,  to  which  they  may  be  closely  related, 
jr  from  which  they  may  be  very  widely  removed  in  their 
zoological  position.  Such  cases  arc  said  to  be  examples  of 
*'  mimicry,"  and  such  animals  arc  said  to  be  "  mimetic." 
One  of  the  best  examples  which  can  be  given  of  this,  is  the 
resemblance  which  certain  of  the  South  American  Butter- 
flies exhibit  to  the  Heliconida^  a  very  brightly-coloured  and 
well-marked  grotip  of  the  butterflies  of  the  same  country. 
Certain  of  the  South  American  butterflies  which  are  in  no 
way  allied  to  the  Heliconida,  and  which  are  also  not  related 
to  each  other,  very  closely  simulate  the  colours  and  mark- 
ings of  the  Heliconida  ;  and  no  doubt  can  be  entertained 
but  that  this  "mimicry"  is  serviceable  to  the  mimics  and 
protects  them  from  injury,  !^^r  Bates,  in  fact,  who  dis- 
covered the  above  facts,  asserts  that  the  Heliconidce^  though' 
very  numerous  and  gaudy  in  their  colouring,  are  not  liable 
to  be  attacked  by  other  animals,  probably  in  consequence 
of  their  possessing  a  strongly  offensive  odour.  The  mimic- 
ing  butterflies,  of  course,  do  not  acquire  this  odour,  but 
they  are  liable  at  a  distance  to  be  mistaken  for  the  distaste- 
ful HeliconidcE^  and  are  thus  doubtless  greatly  protected  from 
the  attacks  of  birds.  Many  other  cases  of  this  kind  of 
mimicry  are  known,  and  it  would  seem  that  in  all  such  cases 
the  mimetic  species  is  protected  from  some  enemy  by  its 
outward  resemblance  to  the  form  which  it  mimics. 

In  another  extensive  group  of  cases,  we  find  an  animal 
imitating,  not  some  other  animal,  but  some  natural  object, 
and  thus  greatly  reducing  its  chances  of  being  detected  by 
its  natural  enemies.  Admirable  examples  of  this  are  afforded 
by  the  insects  known  as  Spectres  {P/iasmides),  some  of 
which  imitate  dried. twigs,  and  are  called  walking-sticks, 
whilst  others  closely  resemble  leaves,  and  are  known  as 
walking-leaves  (fig.  18).  The  advantages  gained  in  these 
cases  are  extremely  obvious,  the  insect  being  plainly  pro- 
tected from  its  foes  by  its  resemblance  to  such  an  object  as 
a  piece  of  dead  wood  or  a  fallen  leaf.     The  closeness  of  the 


MIMICRY. 


53 


resemblance  in  some  of  these  cases  is  most  extraordinary, 
and  no  satisfactory  explanation  of  the  way  in  which  it  is 
produced  has  been  as  yet  advanced.  In  some  cases  the 
resemblance  is  carried  so  far  that  the  animal  not  only  mimics 
some  natural  object,  but  actually  imitates  what  may  be 
termed  the  natural  imperfections  of  the  object.  Thus,  Mr 
Wallace  has  described  a  butterfly  in  which  not  only  does 


Fig.  x8.— W'lking  Leaf  Insect  (JPhyllium). 

the  under  surface  most  closely  resemble  the  leaf  of  a  tree, 
but  "  we  find  representations  of  leaves  in  every  stage  of 
decay,  variously  blotched,  and  mildewed,  and  pierced  with 
holes,  and  in  many  cases  irregularly  covered  with  powdery 
black  dots,  gathered  into  patches  and  spots,  so  closely  re- 
sembling the  various  kinds  of  minute  fungi  that  grow  on 
dead  leaves,  that  it  is  impossible  to  avoid  thinking  at  first 


54  ELEMENTS  OF  BIOLOGY. 

sight  that  the  butterflies  themselves  have  been  attacked  by 
real  fungi."  This  same  eminent  observer  has  pointed  out 
that  the  walking-stick  insects  increase  their  resemblance  to 
twigs  and  branches  by  their  having  the  very  singular  habit 
of  stretching  out  their  legs  in  an  unsymmetrical  and  irregu- 
lar manner. 

V.  Correlation  of  Growth. — This  term  is  employed 
to  designate  the  empirical  law  that  certain  structures,  not 
necessarily  or  usually  connected  together  by  any  discover- 
able link,  invariably  co-exist  or  are  associated  with  one 
another,  and  do  not,  so  far  as  human  observation  goes, 
occur  apart. 

Thus,  all  animals  which  possess  two  condyles  on  the  occi- 
pital bone,  and  possess  non-nucleated  red  blood-corpuscles, 
suckle  Iheir  young.  Why  an  animal  with  only  one  condyle 
on  its  occipital  bone  should  not  suckle  its  young  we  do  not 
know,  and  perhaps  we  shall  at  some  future  time  find  mam- 
mary glands  associated  with  a  single  occipital  condyle. 
Again,  the  feet  are  cleft  in  all  animals  which  ruminate,  but 
not  in  any  other.  In  other  cases  the  correlation  is  even 
more  apparently  lawless,  and  is  even  amusing.  Thus  all,  or 
almost  all,  cats  which  are  entirely  white  and  have  blue  eyes, 
are  at  the  same  time  deaf.  With  regard  to  these  and  similar 
generalisations  we  must,  however^  bear  in  mind  the  follow- 
ing three  points : — 

1.  The  various  parts  of  the  organisation  of  any  animal 
are  so  closely  interconnected,  and  so  mutually  dependent 
upon  one  another,  both  in  their  growth  and  development, 
that  the  characters  of  each  must  be  in  some  relation  to  the 
characters  of  all  the  rest,  whether  this  be  obviously  the  case 
or  not.  •  . 

2.  It  is  rarely  possible  to  assign  any  reason  for  corre- 
lations of  structure,  though  they  are  certainly  in  no  case 
accidental. 

3.  The  law  is  a  purely  empirical  one,  Lnd  i  ^presses  no- 
thing more  than  the  result  of  experience ;  so  that  structiu^es 


ir 


CORRELATION     OF  GROWTH.  55 

which  we  now  only  know  as  occurring  in  association,  may 
ultimately  be  found  dissociated,  and  conjoined  with  other 
structures  of  a  different  character. 

The  term  "  correlation  of  growth  "  may  also  be  applied 
to  those  obscure  relations  which  are  found  to  subsist  be- 
tween certain  organs,  which  have  no  perceptible  connection 
with  one  another,  but  which  are  nevertheless  bound  together 
by  some  very  intimate  "  sympathy."  Thus,  the  full  develop- 
ment of  the  organs  of  reproduction  is  often  accompanied  by 
more  or  less  conspicuous  changes  in  structures  which  would 
appear  to  be  very  remotely  connected  with  the  generative 
functions.  In  man,  for  example,  the  period  of  puberty  is 
marked,  as  a  rule,  by  the  growth  of  hair,  and  by  alterations 
in  the  form  of  the  organs  of  voice.  Still  more  striking  ex- 
amples of  this  obscure  phenomenon  might  be  adduced  as 
regards  the  "  sympathies "  shown  between  certain  organs 
when  diseased.  In  some  of  these  cases,  as  in  the  case  of 
the  "sympathy"  between  the  mammary  glands  and  i^t 
uterus,  it  might  be  said  that  the  two  organs  are  members  of 
one  system ;  but  there  are  instances  (as,  for  example,  in 
"mumps")  in  which  the  sympati using  organs  appear  in  a 
state  of  health  to  be  absolutely  unconnected,  and  to  exercise 
no  influence  upon  each  other. 


CHAPTER    V. 


CLASSIFICATION. 


Classification  is  the  arrangement  of  a  number  of  diverse 
objects  into  larger  or  smaller  groups,  according  as  they  ex- 
hibit more  or  less  likeness  to  one  another.  The  excellence 
of  any  given  classification  will  depend  upon  the  nature  of 
the  points  which  are  taken  as  determining  the  resemblance. 
Systems  of  classification,  in  which  the  groups  are  founded 
upon  mere  external  and  superficial  points  of  similarity, 
though  often  useful  in  the  earlier  stages  of  science,  are 
always  found  in  the  long-run  to  be  inaccurate.  It  is  need- 
less, ill  fact,  to  point  out  that  many  living  beings,  the  struc- 
ture of  which  is  fundamentally  different,  may,  nevertheless, 
present  such  an  amount  of  adaptive  external  resemblance  to 
one  another,  that  they  would  be  grouped  together  in  any 
"  artificial "  classification.  Thus,  to  take  a  single  example, 
the  whale,  by  its  external  characters,  would  certainly  be 
grouped  amongst  the  fishes,  though  widely  removed  from 
them  in  all  the  essential  points  of  its  structure.  "  Natural " 
systems  of  classification,  on  the  other  hand,  endeavour  to 
arrange  animals  into  divisions  founded  upon  a  due  consi- 
deration of  all  the  essential  and  fundamental  points  of 
structure,  wholly  irrespective  of  external  similarity  of  form 
and  habits.  Philosophical  classification  depends  upon  a 
due  appreciation  of  what  constitute  the  true  points  of  differ- 


CLASSIFICATION. 


57 


ence  and  likeness  amongst  animals;  and  we  have  already 
seen  that  these  are  morphological  type  and  specialisation  of 
function.  Philosophical  classification,  therefore,  is  a  formal 
expression  of  the  facts  and  laws  of  Morphology  and  Physio- 
logy. It  follows  that  the  more  fully  the  programme  of  a 
philosophical  and  strictly  natural  classification  can  be  car- 
ried out,  the  more  completely  does  it  afford  a  condensed 
exposition  of  the  fundamental  construction  of  the  objects 
classified.  Thus,  if  the  whale  were  placed  by  an  artificial 
grouping  amongst  the  fishes,  this  would  simply  express  the 
facts  that  its  habits  are  aquatic  and  its  body  fish-like.  When, 
on  the  contrary,  we  obtain  a  natural  classification,  and  we 
learn  that  the  whale  is  placed  amongst  the  Mammalia,  we 
then  know  at  once  that  the  young  whale  is  born  in  a  com- 
paratively helpless  condition,  and  that  its  mother  is  provided 
with  special  mammary. glands  for  its  support;  this  express- 
ing a  fundamental  distinction  from  all  fishes,  and  being 
associated  with  other  equally  essential  correlations  of  struc- 
ture. 

The  entire  animal  kingdom  is  primarily  divided  into 
some  half-a-dozen  great  plans  of  structure,  the  divisions 
thus  formed  being  called  "  sub-kingdoms,"  The  sub-king- 
doms are,  in  turn,  broken  up  into  classes,  classes  in^;  i  orders, 
orders  into  families,  families  into  genera,  and  genera  into 
species.  We  shall  examine  these  successively,  commencing 
with  the  consideration  of  a  species,  since  this  is  the  zoolo- 
gical unit  of  which  the  larger  divisions  are  made  up. 

Species. — No  term  is  more  difficult  to  define  than  "  spe- 
cies," and  on  no  point  are  zoologists  more  divided  than  re 
to  what  should  be  understood  by  this  word.  Naturalists, 
in  fact,  are  not  yet  agreed  as  to  whether  the  term  species 
expresses  a  real  and  permanent  distinction,  or  whether  it  is 
to  be  regarded  merely  as  a  convenient,  but  not  immutable, 
abstraction,  the  employment  of  which  is  necessitated  by 
the  requirements  of  classification. 

By  Buffon  "species"  is  defined  as  "a  constant  succession 


5$  ELEMENTS  OF  BIOLOGY. 

of  individuals*  similar  to  and  capable  of  reproducing'  each 
other." 

De  CandoUe  defines  species  as  an  assemblag^of  all  those 
individuals  which  resemble  each  other  more  than  they  do 
others,  and  are  able  to  reproduce  their  like,  doing  so  by  the 
generative  process,  and  in  such  a  manner  that  they  may  be 
supposed  by  analogy  to  have  all  descended  from  a  single 
being  or  a  single  pair. 

M.  de  Quatrefages  defines  species  as  "an  assemblage 
of  individuals,  more  or  less  resembling  one  another,  which 
are  descended,  or  may  be  regarded  as  being  descended, 
from  a  single  primitive  pair  by  an  uninterrupted  succession 
of  families." 

Miiller  defines  species  as  "  a  living  form,  represented  by 
individual  beings,  which  reappears  in  the  product  of  gener- 
ation with  certain  invariable  characters,  and  is  constantly 
reproduced  by  the  generative  act  of  similar  individuals." 

According  to  Woodward,  "all  the  specimens,  or  indi- 
viduals, which  are  so  much  alike  that  we  may  reasonably 
believe  them  to  have  descended  from  a  common  stock,  con- 
stitute a  species.'^ 

From  the  above  definitions  it  will  be  at  once  evident  that 
there  are  two  leading  ideas  in  the  minds  of  zoologists  when 
they  employ  the  term  species ;  one  of  these  being  a  certain 
amount  of  resemblance  between  individuals,  and  the  other 
being  the  proof  that  the  individuals  so  resembling  each 
other  have  descended  from  a  single  pair,  or  from  pairs  ex- 
actly similar  to  one  another.  The  characters  in  which  indi- 
viduals must  resemble  one  another  in  order  to  entitle  them 
to  be  grouped  in  a  separate  species,  according  to  Agassiz, 
"  are  only  those  determining  size,  proportion,  colour,  habits, 


|sj  *  In  using  the  term  "individual,"  it  must  be  borne  in  mind  that  the 

'•zoological  individual"  is  meant;  that  is  to  say,  the  total  result  of  the 
development  of  a  single  ovum,  as  will  be  hereafter  explained  at  greater 
length. 


CLASSIFICATION.  59 

and  relations  to  surrounding  circumstances  and  external 
objects." 

On  a  closer  examination,  however,  it  will  be  found  that 
these  two  leading  ideas  in  the  definition  of  species — external 
resemblance  and  community  of  descent — are  both  defective, 
and  liable  to  break  down  if  rigidly  applied.  Thus,  there 
are  in  nature  no  assemblages  of  plants  or  animals,  usually 
grouped  together  into  a  single  species,  the  individuals  of 
•  which  exactly  resemble  one  another  in  every  point.  Every 
naturalist  is  compelled  to  admit  that  the  individuals  which 
compose  any  so-called  species,  whethei  of  plants  or  animals, 
differ  from  one  another  to  a  greater  or  less  extent,  and  in 
respects  which  may  be  regarded  as  more  or  less  important. 
The  existence  of  ^uch  individual  differences  is  attested  by 
the  universal  employment  of  the  terms  "varieties"  and 
" races."  Thus,  a  "  variety"  comprises  all  those  individuals 
which  possess  some  distinctive  peculiarity  in  common,  but 
do  not  differ  in  other  respects  from  another  set  of  indi- 
viduals sufficiently  to  entitle  them  to  take  rank  as  a  separate 
species.  A  "  race,"  again,  is  simply  a  permanent  or  "  per- 
petuated "  variety.  The  question,  however,  is  this — Hcav 
far  may  these  differences  amongst  individuals  obtain  with- 
out necessitating  their  being  placed  in  a  separate  species  ? 
In  other  words :  How  great  is  the  amount  of  individual 
difference  which  is  to  be  considered  as  merely  "  z'^r/V/'tf/," 
and  at  what  exact  point  do  these  dififerences  become  of 
'■^specific'"  value?  To  this  question  no  answer  can  be  given, 
since  it  depends  entirely  upon  the  weight  v/hich  different 
naturalists  would  attach  to  any  given  individual  difference.* 
Distinctions  which  appear  to  one  observer  as  sufficiently 
great  to  entitle  the  individuals  possessing  them  to  be  grouped 
as  a  distinct  species,  by  another  are  looked  upon  as  simply 

*  As  an  example  of  this,  it  is  sufficient  to  allude  to  the  fact  that  hardly 
any  two  botanists  agree  as  to  the  number  of  species  of  Willows  and 
Brambles  in  the  British  Isles.  What  one  observer  classes  as  mere 
varieties,  another  regards  as  good  and  distinct  species. 


t 


60  ELEMENTS  OF   BIOLOGY. 

of  varietal  value ;  and,  in  the  nature  of  the  case,  it  seems  im- 
possible to  lay  down  any  definite  rules.  To  such  an  extent 
do  individual  differences  sometimes  exist  in  particular 
gc  nera — termed  "  protean  "  or  "  polymorphic  "  genera — that 
the  determination  of  the  different  species  and  varieties  be- 
comes an  almost  hopeless  task. 

Besides  the  individual  differences  which  ordinarily  occur 
in  all  species,  other  cases  occur  in  which  a  species  consists 
normally  and  regularly  of  two  or  even  three  distinct  forms, 
which  cannot  be  said  to  be  mere  varieties,  since  no  inter- 
mediate forms  can  be  discovered.  When  two  such  distinct 
forms  exist,  the  species  is  said  to  be  "  dimorphic,"  and  when 
three  are  present  it  is  called  "  trimorphic."  Thus  in  di- 
morphic plants  a  single  species  is  composed  of  two  distinct 
forms,  similar  to  one  another  in  all  respects  except  in  their 
reproductive  organs,  the  one  form  having  a  long  pistil  and 
short  stamens,  the  other  a  short  pistil  with  long  stamens. 
In  trimorphic  plants  the  species  is  composed  of  three  such 
distinct  forms,  which  differ  in  like  manner  in  the  conforma- 
tion of  their  reproductive  organs,  though  they  are  otherwise 
undistinguishable. — (Darwin.)  Similar  cases  are  known  in 
animals,  but  in  them  the  differences,  though  apparently  con- 
nected with  reproduction,  are  not  confined  to  the  reproduc- 
tive organs.  Thus  the  females  of  certain  butterflies  normally 
appear  under  two  or  three  entirely  different  forms,  not  con- 
nected by  any  intermediate  links,  and  the  same  thing  occurs 
in  some  of  the  Cnistacea. 

As  regards,  therefore,  the  first  point  in  the  definition  of 
species — namely,  the  external  resemblance  of  assemblages 
of  individuals — we  are  forced  to  conclude  that  no  two  indi- 
viduals are  exactly  alike ;  and  that  the  amount  and  kind  of 
external  resemblance  which  constitutes  a  species  is  not  a 
precise  and  invariable  quantity,  but  depends  upon  the  value 
attached  to  particular  characters  by  any  given  observer. 

The  second  point  in  the  definition  of  species — namely, 
community  of  descent — is  hardly  in  a  more  satisfactory  con- 


! 


CLASSIFICATION.  6l 

dition,  since  the  descent  of  any  given  series  of  individuals 
from  a  single  pair,  or  from  pairs  exactly  similar  to  one 
another,  is  at  best  but  a  probability,  and  is  in  no  case  cap- 
able of  proof.  In  the  case  of  the  higher  animals  it  can 
doubtless  be  shown  that  certain  assemblages  of  individuals 
possess  amongst  themselves  the  power  of  fecundation  and 
of  producing  fertile  progeny,  and  that  this  power  does  not 
extend  to  the  fecundation  of  individuals  belonging  to  another 
different  assemblage.  Amongsl  the  higher  animals,  "crosses" 
or  "  hybrids  "  can  only  be  produced  between  closely-allied 
species ;  and  when  produced  they  are  sterile,  and  are  not 
capable  of  reproducing  their  like.  In  these  cases,  there- 
fore, we  may  take  this  as  a  most  satisfactory  element  in  the 
definition  of  "  species."  The  sterility,  however,  of  hybrids 
is  not  universal,  even  amongst  the  higher  animals ;  and 
amongst  plants  no  doubt  can  be  entertained  but  that  the 
individuals  of  species  universally  admitted  to  be  distinct  are 
capable  of  mutual  fertilisation ;  the  hybrid  progeny  thus 
produced  being  likewise  fertile,  and  capable  of  reproducing 
similar  individuals.  That  this  fertility  is  often  irregular, 
and  may  be  destroyed  in  a  few  generations,  admits  of  ex- 
planation, and  hardly  alters  the  significance  of  these  un- 
doubted facts. 

Upon  the  whole,  then,  it  seems  in  the  meanwhile  safest 
to  adopt  a  definition  of  species  which  implies  no  theory,  and 
does  not  include  the  belief  that  the  term  necessarily  expresses 
a  fixed  and  permanent  quantity.  Species,  therefore,  may 
be  defined  as  an  assemblage  of  individuals  which  resemble  each 
other  in  their  essential  characters,  are  able,  directly  or  indirectly, 
to  produce  fertile  individuals,  and  which  do  not  {as  far  as 
human  observation  gor^)  give  rise  to  individuals  which  vary 
from  the  general  type  through  more  than  certain  definite  lifnits. 
The  production  of  occasional  monstrosities  does  not,  of 
course,  invalidate  this  definition. 

Genus  is  a  term  applied  to  groups  of  species  which  pos- 
sess a  community  of  essential  details  of  structure.  A  genus 
4 


62  ELEMENTS  OF  BIOLOGY. 

may  include  a  single  species  only,  in  cases  where  the  com- 
bination of  characters  which  make  up  the  species  are  so 
peculiar  that  no  other  species  exhibits  similar  structural 
characters;  or,  on  the  other  hand,  it  may  contain  many 
hundreds  of  species. 

Families  are  groups  of  genera  which  agree  in  their  general 
characters.  According  to  Agassiz,  they  are  divisions  founded 
upon  peculiarities  of  "form  as  determined  by  structure." 

Orders  are  groups  of  families  related  to  one  another  by 
structural  characters  common  to  all. 
t  Classes  are  larger  divisions,  comprising  animals  which  are 

formed  upon  the  same  fundamental  plan  of  structure,  but 
differ  in  the  method  in  which  the  plan  is  executed  (Agas- 
siz). 

Sub-kingdoms  are  the  primary  divisions  of  the  animal  king- 
dom, which  include  all  those  animals  which  are  formed 
upon  the  same  structural  or  morphological  type,  irrespective 
of  the  degree  to  which  specialisation  of  functions  may  be 
carried. 

IMPOSSIBILITY    OF   A   LINEAR   CLASSIFICATION. 

It  has  sometimes  been  thought  that  the  animal  kingdom 
can  be  arranged  in  a  linear  series,  every  member  of  the 
series  being  higher  in  point  of  organisation  than  the  one 
below  it.  As  we  have  seen,  however,  the  status  of  any  given 
animal  depends  upon  two  conditions — one  its  morphologi- 
cal type,  the  other  the  degree  to  which  specialisation  of 
functions  is  carried.  Now,  if  we  take  two  animals,  one  of 
which  belongs  to  a  lower  morphological  type  than  the  other, 
no  degree  of  specialisation  of  functions,  however  great,  will 
place  the  former  above  the  latter,  as  far  as  its  type  of  struc- 
ture is  concerned,  though  it  may  make  the  former  a  more 
\  highly  organised  animal.      Every  Vertebrate   animal,   for 

example,  belongs  to  a  higher  morphological  type  than  every 
|i  Mollusc ;  but  the  higher  Molluscs,  such  as  cuttle-fishes,  are 

'  much  more  highly  organised,  as  far  as  their  type  is  con- 


CLASSIFICATION.  G3 

ccrned,  than  arc  the  lowest  Vcrtcbrata.  In  a  linear  classifi- 
cation, therefore,  the  cuttle-fishes  should  be  placed  above 
the  lowest  fishes — such  as  the  lancelet — in  spite  of  the  fact 
that  the  type  upon  which  the  latter  are  constructed  is  by 
far  the  highest  of  the  two. 

It  is  obvious,  therefore,  that  a  linear  classification  is  not 
possible,  since  the  higher  members  of  each  sub-kingdom  are 
more  highly  organised  than  the  lower  forms  of  the  next 
sub-kingdom  in  the  series,  at  the  same  time  that  they  are 
constructed  upon  a  lower  morphological  type. 

In  the  words  of  Professor  Allen  Thomson,  "  it  has  become 
more  and  more  apparent  in  the  progress  of  morphological 
research,  that  the  different  groups  form  ciicles  which  touch 
one  another  at  certain  points  of  greatest  resemblance,  rather 
than  one  continuous  line,  or  even  a  number  of  lines  which 
partially  pass  each  other."  In  the  same  way  the  highest 
vegetables  do  not  approximate  to,  or  graduate  into,  the 
lowest  animals;  but  "each  kingdom  presents,  as  it  were, 
a  radiating  expansion  into  groups  for  itself,  so  that  the  rela- 
tions of  the  two  kingdoms  might  be  represented  by  the 
divergence  of  lines  spreading  in  two  different  directions  from 
a  common  point." 


CHAPTER    VI. 

ELEMENTARY    CHEMISTRY    OF    LIVING    BEINGS. 

A  ROUGH  analysis  of  any  living  body,  whether  animal  or 
vegetable,  would  show  that  it  consists  of  water,  certain  or- 
ganic compounds,  and  certain  inorganic  matter.  Ky  a 
gentle  heat  the  water  may  be  expelled,  when  it  would  be 
found  that  the  body  experimented  on  would  have  lost, 
speaking  generally,  from  seventy  to  ninety  per  cent  of  its 
weight.  Living  matter,  therefore,  is  very  largely  made  up 
of  water,  which,  indeed,  is  an  absolute  necessity  for  the  per- 
formance of  all  vital  actions.  After  ^riving  off  the  water,  if 
a  strong  heat  be  applied,  it  would  be  found  that  a  certain 
proportion  of  the  dried  tissue  would  be  burnt  and  would  be 
completely  dissipated.  In  this  way  we  should  eliminate  a 
certain  quantity  of  organic  compounds,  which  would  differ 
according  to  the  character  of  the  tissue  we  were  dealing 
with,  and  into  the  nature  of  which  we  shall  inquire  immedi- 
ately. Lastly,  there  would  remain  a  small  proportion  of 
mineral  or  inorganic  matter  which  constitutes  the  "ash," 
and  which  would  not  be  dissipated  or  affected  by  the  incin- 
eration. The  average  amount  of  ash  in  animal  tissues  is 
about  three  per  cent,  but  in  the  case  of  vegetables  the  min- 
eral constituents  may  be  present  in  larger  proportions  than 
this. 

A  living  body,  then,  may  be  said  to  consist  of  water,  cer* 


CIIKMISTRY  OF  ANIMALS.  6$ 

tain  complex  organic  compounds,  and  a  small  proportion 
of  certain  mineral  or  inorganic  substances.  The  presence 
of  all  these  three  constituents  appears  to  be  essential  to  the 
existence  of  living  matter,  and  vital  action  can  not  appar- 
ently be  carried  on  in  the  absence  of  any  one  of  the  three. 
It  is  to  be  remembereu,  however,  that  though  we  can  make 
such  a  rough  analysis  as  the  above  of  the  matter  of  living 
beings,  we  know  really  very  little  of  the  mode  in  which 
these  constituents  are  combined  in  the  living  body.  Thus 
it  is  uncertain  how  much  of  the  water  is  in  a  state  of  chem- 
ical combination  with  other  constituents  of  the  tissues;  and 
we  are  still  more  ignorant  of  the  exact  mode  in  which  the 
mineral  or  inorganic  constituents  occur.  It  is  certain,  how- 
ever, that  some,  at  any  rate,  of  the  mineral  substances  are 
chemically  combined  with  the  organic  compounds ;  wh'"at 
it  is  quite  certain  that  Ifof/i  groups  of  substances  are  essen- 
tial to  life. 

In  the  following,  a  very  brief  outline  will  be  given  of  the 
more  important  facts  as  to  the  elementary  chemistry  of 
animals  and  plants  respectively  : — 

CHEMISTRY   OF   ANIMALS. 

The  number  of  elements  which  have  been  recognised 
in  animal  bodies  is  not  very  large,  the  chief,  if  not  the 
only  ones,  being  carbon,  hydrogen,  oxygen,  nitrogen,  sul- 
phur, phosphorus,  chlorine,  fluorine,  calcium,  magnesium, 
aluminium,  potassium,  sodium,  iron,  manganese,  copper, 
and  silicon.  The  first  four  elements  of  this  list  are  some- 
tines  spoken  of  as  the  "  essential  elements,"  as  they  occur 
in  most  tissues ;  whilst  the  remainder  are  very  improperly 
t<i\  Tied  the  "  incidental  elements,"  as  occurring  only  in 
small  quantities  and  in  special  tissues.  Some,  however,  of 
these  "  incidental  elements "  are  essential  constituents  of 
the  compounds  formed  by  the  so-called  "  essential  ele- 
ments," and  most  of  them  are  just  as  necessary  to  life  as 
the  latter  are. 


:l 


w 


66  i:r,KMKNTs  of  biology. 

Little  need  be  said  here  »is  to  the  occurrence  of  the 
80-calIed  "  incidental  elements."  Sulphur  occurs  in  albu- 
men, as  one  of  its  constituents,  and  phosphorus  is  found  in 
nervous  matter,  and  is  largely  present  in  bone  (as  i)hosphatc 
of  lime).  Chlorine  occurs  (as  chloride  of  sodium)  in  ani- 
mal juices,  and  in  gastric  juice  (as  hydrochloric  acid). 
Fluorine  (in  the  form  of  fluoride  of  calcium)  occurs  in  the 
teeth.  Silicon,  aluminium,  calcium,  and  magnesium,  occur 
in  the  teeth  and  bones.  Sodium  and  potassium  are  found 
in  the  blood,  and  the  former  is  chemically  combined  with 
albumen  in  its  soluble  state.  Iron  is  found  in  the  colouring- 
matter  of  the  blood,  and,  unlike  the  other  metals,  is  pro- 
bably present  in  an  uncombined  condition.  Manganese 
has  been  detected  in  hair,  and  is  also  stated  to  occur  in  the 
blood.  Lastly,  copper  is  found  in  the  liver  and  in  bile,  and 
in  some  colouring-matters. 

The  "  essential "  elements,  carbon,  hydrogen,  oxygen,  and 
nitrogen,  occur  united  with  compounds,  which,  *'  from  their 
being  supposed  to  stand,  in  order  of  simplicity,  nearest  to 
the  elements,"  are  called  proximate  principles.  In  other 
words,  these  four  elements  form  a  series  of  compounds 
which  have  a  definite  chemical  composition,  which  may  be 
obtained  in  an  isolated  condition  from  animal  and  vegetable 
bodies  after  death ^  and  which  in  some  cases  can  be  artifi- 
cially built  up  out  of  inorganic  materials  in  the  laboratory 
of  the  chemist.  The  "  proximate  compounds  "  of  both  ani- 
mals and  plants  may  be  divided  into  two  groups,  termed 
non- nitrogenous  and  nitrogenous^  according  as  they  consist  of 
carbon,  hydrogen,  and  oxygen  alone,  or  contain  nitrogen  in 
addition  to  these  three  elements. 

The  non-nitrogenous  compounds  of  animals  are  the  various 
Fats.  These  consist  of  carbon,  hydrogen,  and  oxygen, 
combined  in  such  proportions  that  the  oxygen  would  be 
insufficient  to  form  water  with  the  hydrogen  or  carbonic 
acid  with  the  carbon.  The  exact  functions  of  the  fats  in 
the  animal  economy  cannot  be  said  to  have  been  as  yet  de- 


CHEMISTRY  OF  ANIMALS.  67 

tcrmincd  in  a  thoroughly  satisfactory  manner.  Fats  occur 
in  most  animal  tissues  and  fluids,  and  in  many  cases  they 
are  certainly  not  unnecessary  or  superfluous  constituents. 
There  can  also  be  no  doubt  but  that  the  fats  are  largely 
instrumental  in  maintaining  the  temi)erature  of  the  body. 

The  nitn\i:;t'fious  compounds  of  animals  are  numerous,  but 
the  three  most  important  are  albumen,  fibrine,  and  cascine. 

Albumen  is  a  comjjound  of  carbon,  hydrogen,  oxygen, 
nitrogen,  and  sulphur,  but  in  its  soluble  form  it  is  combined 
with  some  salt  of  sodium.  In  its  soluble  state,  albumen  is 
a  colourless,  tasteless,  glairy  fluid,  which  *'  coagulates "  or 
becomes  solid  at  a  temperature  of  about  150°  Fahr.,  is  pre- 
cipitated by  all  the  mineral  acids  (except  tribasic  phos- 
phoric acid),  and  is  not  precipitated  by  any  of  the  vegetable 
acids  (except  tannic  acid).  Albumen  is  also  thrown  down 
from  its  solutions  by  alum,  corrosive  sublimate,  sulphate  of 
copper,  acetate  of  lead,  creasote,  and  alcohol.  Albumen  is 
found  in  the  blood,  and  in  most  of  the  animal  fluids,  and 
also  in  some  tissues ;  and  white  of  egg  is  almost  wholly 
composed  of  it. 

Fibrins  is  very  closely  allied  to  albumen,  and  is  best 
known  as  occurring  in  a  fluid  form  in  the  blood.  It  also 
occurs,  in  a  slightly  modified  state,  in  muscle.  It  has  the 
power,  when  removed  from  the  body,  and  sometimes  whilst 
still  within  the  body,  of  spontaneously  solidifying  or  coag- 
ulating. When  coagulated,  it  is  almost  undistinguishable 
chemically  from  coagulated  albumen. 

Caseine  is  an  albuminous  body  which  occurs  abundantly 
in  milk.  It  differs  from  albumen  in  not  being  coagulated 
by  heat  alone,  but  in  being  precipitated  from  its  solutions 
by  acetic  acid. 

The  eminent  chemist  Mulder  held  the  opinion  that  albu- 
men, fibrine,  and  caseine,  with  the  similar  bodies  found  in 
vegetables,  are  compounds  of  a  substance  which  he  named 
"  proteine "  with  sulphur  and  phosphorus.  He  further 
believed  that  "proteine"  consisted  of  the  four  essential 


II 


i!il! 


68  ELEMENTS  OF  BIOLOGY. 

elements,  carbon,  hydrogen,  oxygen,  and  nitrogen,  alone. 
Other  good  authorities  deny  the  existence  of  any  such  base 
as  proteine.  Nevertheless,  it  is  a  common  and  often  a  very 
convenient  practice  to  speak  of  the  various  albuminoid 
substances  of  animals  or  vegetables  a''  "  pioteids,"  or  "pro- 
teine compounds." 


CHEMISTRY   OF   VEGETABLES. 

The  organic  substances  which  compose  the  tissues  of 
I)lants,  as  in  the  case  of  those  of  animals,  may  be  divided 
into  a  non-nitrogenous  and  a  nitrogenous  group,  according 
as  they  consist  of  carbon,  hydrogen,  and  oxygen  alone,  or 
contain  nitrogen  in  addition  to  these  three  elements.  The 
chief  difference  to  be  noted  between  animals  and  vegetables, 
as  regards  their  chemical  composition,  concerns  the  pro- 
portion borne  by  the  nitrogenous  substances  to  the  non- 
nitrogenous.  In  bclh  kingdoms  we  find  "pioxima.e  prin- 
ciples "  which,  if  not  actually  identical,  at  any  rate  represmt 
each  other :  but  there  is  a  considerable  distinction  in  the 
relative  amount  of  the  two  groups  of  compounds  in  a  plant 
as  compared  with  an  animal.  Animal  bodies  exhibit  a, 
marked  predominance  of  albuminoid  or  nitrogenous  com- 
pounds over  the  fatty  or  non-nitrogenous  compounds. 
Plants,  on  the  other  hand,  are  mainly  composed  of  non- 
nitrogenous  compounds,  and  they  are,  comparatively  speak- 
ing, poor  in  albuminous  or  nitrogenous  matter. 

The  chief  non-nitrogmous  principles  of  plants  are  starch, 
cellulose  and  sugar,  all  of  which  differ  from  the  fatty  com- 
pounds of  animals  in  the  fact  that  the  oxygen  is  present  in 
sufficient,  quai^tity  to  form  water  with  the  hydrogen.  Plants, 
ho-vevcr,  are  by  no  means  destitute  of  non-nitrogenous  sub- 
stances in  which  the  proportion  of  oxygen  is  less  than  this, 
or  in  which,  indeed,  oxygen  is  wholly  absent. 
I  Starch  is  composed  of  Carbon,  Hydrogen,  and  Oxygen, 

with  the  formula  CjgHjoOjo.    It  occurs  plentifully  in  vege- 
table tissues,  especially  in  seeds,  fruits,  stems,  and  roots;  and 


CHEMISTRY   OF   VEGETAP,LES.  69 

it  is  recognised  by  the  addition  of  iciine,  when  a  blue  colour 
is  produ'^ed,  owing  to  the  formation  of  a  bhie  iodide  of 
starch.  Starch,  as  such,  is  not  sohible  in  the  fluids  of  the 
body,  but  it  is  readily  rendered  soluble  by  the  action  of  cer- 
tain bodies  of  the  nature  of  ferments.  Starch  is  also  ren- 
dered soluble  by  the  action  of  prolonged  heat,  or  dilute 
sulphuric  acid,  when  it  is  converted  into  the  gummy  sub- 
stance known  as  "  Dextrine  "  or  "  British  Gum." 

Cellulose  is  largely  present  in  plants,  and  enters  to  a  great 
extent  into  the  composition  of  the  cells  and  vessels  of  all 
vegetable  tissues.  Though  allied  to  starch,  it  differs  from 
it  in  some  important  respects,  esjiecially  in  the  fact  that  it 
gives  no  blue  colour  on  the  addition  of  iodine.  When 
cellulose,  however,  is  digested  for  a  short  time  in  sulphtiric 
acid,  it  is  partially  converted  into  starch,  as  shown  by  the 
fact  that  iodine  will  then  produce  the  fine  blue  colour  of  the 
iodide  of  ::  .arch.  The  wood/  tissue  which  is  deposited  in 
the  hard  parts  of  plants,  and  waich  is  often  called  "  Lignine," 
may  be  regarded  as  probably  a  modification  of  cclkilose. 

Sugar  is  present  in  almost  all  plants,  chiefly  in  their  sap. 
The  two  most  important  varieties  of  vegetable  sugar  are 
"cane-sugar"  and  "grape-sugar,"  both  of  which  arc  capable 
of  crystallising. 

The  nitrogenous  compounds  of  pkmts  need  little  more 
than  mention,  as  they  do  not  appear  to  dift'er  in  any  essen- 
tial respect  from  the  albuminous  compounds  of  animals. 
The  most  important  is  "gluten,"  which  occurs  abundantly 
in  the  seeds  of  Cereals  and  in  the  juices  of  many  plants. 
It  is  nearly  allied  to  the  "  fibrine  "  of  animals,  and  has  the 
power  of  spontaneously  coagulating  from  its  solutions.  The 
juices  of  many  plants  also  contain  a  proteine  compound 
which  is  coagulated  by  heat,  and  which  appears  to  be  iden- 
tical with  the  albumen  of  animals.  Lastly,  in  the  seeds  of 
peas,  beans,  and  other  leguminous  plants,  there  is  found  a 
substance  which  is  termed  "  legumine,"  an  j  which  appears 
to  be  nearly  allied  tc  the  ca«eine  of  milk. 


J! 


«, 


4 


CHAPTER   VII. 

ELEMENTARY   STRUCTURE   OF   LIVING  BODIES. 
PROTOPLASM    OR    BIOPLASM. 

As  has  been  before  mentioned,  the  presence  of  an  albu- 
minoid substance,  or  "  physical  basis,"  appears  to  be  abso- 
lutely essential  to  the  manifestation  of  vital  action ;  but  it  is 
by  no  means  absolutely  necessary  that  this  substance  should 
be  so  "differentiated"  as  to  exhibit  anything  that  would 
properly  be  called  structure.  All  the  phenomena  of  life  seem 
capable  of  manifesting  themselves  through  the  medium  of 
albuminoid  matter,  in  which,  at  most,  very  minute  particles 
or  molecules  are  developed.  This  albuminoid  matter  is  the 
"  protoplasm  "  of  Professor  Huxley  and  other  writers  \  but 
it  is  better  designated  by  the  name  of  "  bioplasm,"  applied 
to  it  by  Dr  Beale.  In  the  case,  then,  of  some  of  the  lower 
fonns  of  animal  life,  such  as  the  Foramini/era  (fig.  i),  or  the 
still  more  degraded  Monera  of  Haeckel,  the  organism  con- 
sists wholly  of  bioplasmic  matter,  which  may  fairly  be  called 
"  structureless,"  since  it  exhibits  nothing  in  the  way  of 
definite  organs,  and  has,  at  most,  a  number  of  small  par- 
ticles or  molecules  scattered  through  it.  Nevertheless,  the 
animal  performs  all  the  functions  of  nutrition  and  reproduc- 
tion, and  exhibits  all  the  essential  phenomena  of  life. 

Protoplasmic  matter,  or  "  bioplasm,"  constitutes  the  basis 
of  the  ovum  of  both  animals  and  plants ;  but  there  are  none 


PROTOPLASM.  71 

of  the  latter  in  which  we  have  such  an  elementary  state  of 
things  as  in  the  Foraniinifcra.  Even  in  the  very  lowest  of 
the  plants  the  living  matter  of  the  embryo  is  bounded  in  the 
adult  by  a  definite  wall,  thus  becoming  what  will  be  imme- 
diately described  as  a  ''cell." 

Bioplasmic  matter  is  colourless,  transparent,  and  ap- 
parently wholly  destitute  of  structure.  It  has  the  property, 
as  shown  by  Dr  Beale,  of  being  deeply  tinged  by  an  am- 
moniacal  solution  of  carmine,  whereby  its  presence  is  readily 
detected.  In  all  cases  it  has  the  power  of  spontaneous 
movement,  as  may  be  well  shown  by  an  examination  of 
such  a  minute  mass  of  bioplasm  as  is  afforded  by  an  Amoeba 
or  a  mucus-corpuscle.  In  all  cases  the  movements  of  bio- 
plasmic matter,  when  unrestricted  by  any  imprisoning  en- 
velope, are  similar  in  kind  to  those  of  the  ordinary  Amoebae; 
that  is  to  say,  the  bioplasm  has  the  power  of  extending 
itself  in  all  directions  in  the  form  of  mutable  processes, 
which  can  be  withdrawn  at  will.  These  movements  are 
often  spoken  of  as  instances  of  "  contractility ; "  but  the 
term  is,  perhaps,  hardly  a  suitable  one,  as  it  implies  that 
these  movements  are  identical  in  kind  with  the  contractions 
of  a  muscle.  Lastly,  it  has  recently  been  shown  that  in 
some  cases  minute  masses  of  bioplasm  have  the  extraordi- 
nary power  of  passing,  or,  as  it  ■^QXQ,flo7ving,  through  closed 
membranes,  without  thereby  losing  their  identity  or  form. 
Thus  it  has  been  shown  that  the  white  corpuscles  of  the 
blood  have  the  power  of  passing  through  the  delicate  walls 
of  the  capillary  blood-vessels,  and  of  thus  obtaining  access 
to  the  tissues. 

The  very  minute  particles  which  are  known  as  "  mole- 
cules "  do  not  require,  as  thought  by  some,  to  be  considered 
apart  from  bioplasm.  The  physical  basis  of  life  seems  to 
be  structureless,  and  apparently  homogeneous  bioplasmic 
matter.  The  simplest  forms  of  living  matter,  however,  at 
an  early  stage  exhibit  extremely  minute  solid  particles  or 
molecules.    The  first  forms  of  life,  also,  which  arc  developed 


72 


ELExMENTS  OF  BIOLOGY. 


in  infusions  containing  organic  matter,  are,  as  we  shall  sub- 
sequently see,  inconceivably  minute  molecules.  In  this,  as 
in  other  cases,  the  molecules  are  to  be  regarded  in  all  pro- 
bability as  being  small  masses  or  centres  of  bioplasm,  which 
may  or  may  not  be  surrounded  by  a  proper  wall. 


5    ! 

I 


CELLS. 

If  we  regard  a  little  mass  or  spherule  of  bioplasmic  matter 
as  being  the  primitive  and  simplest  life-element,  it  never- 
theless is  very  rare  to  find  this  primordial  condition  un- 
complicated or  retained  throughout  life.  In  the  Foramini- 
fcra  and  Monera  we  may,  perhaps,  consider  that  we  have 
the  nearest  approach  to  this  elementary  state  of  things, 
since  the  body  in  these  degraded  organisms  consists  simply 
of  a  mass  t  f  structureless  bioplasm,  in  which  there  is  no 
differentiativ^n  into  definite  parts.  In  the  majority  of  cases, 
however,  changes  take  place  in  the  primitive  mass  of  bio- 
plasm, by  which  it  is  converted  into  what  is  known  as  a 
cell.  In  some  plants,  hence  termed  *'  unicellular,"  a  single 
cell  constitutes  the  entire  organism.     In  such  cases,  as  in 

the  Yeast-plant  (fig.  19),  a 
complete  individual  may 
be  regarded  as  composed 
of  one  cell,  since  in  this 
resides  the  power  of  both 
nutrition  and  reproduc- 
tion. In  the  majority  of 
cases,  however,  the  organ- 
ism is  composed  of  a  con- 
Fig.  19.— Cells  of  the  Yeast -plant  {torula  gericS  of  CClls,  Cach  of 
cerevisue),  greatly  magnified.     The  shaded        ,  .    , 

portions  represent  the   bioplasm,  coloured    whlch    CnjOyS    tO  a  Certain 
by  carmine.  ,.-  .    . 

extent  a  life  of  its  own, 
whilst  its  existence  is,  nevertheless,  bound  up  with  that  of 
the  whole.  Not  only  is  this  the  case,  but  h\  mii  jy  instances 
the  cells  which  form  the  organism  are  so  modified  that  they 
constitute  special  tissues,  such  as  muscular  tissue,  cartilage, 


^  <@) 


CELLS.  73 

tendon,  bone,  &c.  It  would  be  wholly  loreign  to  the  pur- 
pose of  this  work  to  describe  the  various  tissues  which  enter 
into  the  composition  of  an  animal  or  plant,  and  it  will  be 
sufficient  to  describe  briefly  the  stmcture  of  a  cell. 

The  structural  elements  which  compose  a  typical  cell 
(fig.  20)  are  the  following  : — 

I.  The  Cell-wall. — This  is  the  outer  layer  or  membrane  by 
which  the  cell  is  bounded  (fig.  20,  a).  It  does  not  appear 
to  be  essential   to  the   existence 

a 

of  a  cell,  and  certainly  is  not  the  j^'^--'''' 

agent  by  which  cellular  activity  is  j^^''^     ■%<. * 

manifested.     On  the  contrary,  the  i^l       M\        ^ 

cell-wall  appears  to  be  formed  by  II  ^        W^ss:=J^    '^ 

the  transformation,  or  partial  death,  ^ps=;Jlf^''^\ 

so  to  speak,  of  the  outermost  por-  u^      ^j            )^ 

tion  of  the  cell-contents.     Thus,  \l          Iw           nV"" 

on  the  view   advocated   by   that  \    Q  jj\         j) 

eminent  microscopist,   Dr  Lionel  \5=i^    x^;;.;^ 

Beale,    we     must    regard    the     cell-     Fig.  20.— Four  cells  from  the  noto- 
,,  1       /-  1  •    1  chord  of  the  Lamprey.     Greatly 

wall  as  composed  of  matter  which      magnified.    (After  Todd  and 

<  1,1  1         11, 1  '.    t        Bowman.)    «  Cell-wall ;  ^  Cell- 

haS     passed    through    all    the    vital        contents;  c   Nucleus  with   nu- 

changes  of  which  it  is  capable —  cieoius. 
matter  which  is  fanned^  not  formative,  or,  in  other  words, 
matter  which  is  more  or  less  nearly  dead.  The  vit;  1  activity 
of  a  cell  is  therefore  more  or  less  directly  dependent  upon 
the  nature  of  the  cell-wall ;  and  the  thicker  and  more  devel- 
oped the  cell-wall  becomes,  the  less  efficient  is  the  cell. 
The  actual  composition  of  the  cell-wall  differs  in  different 
cases.  In  animal  cells  it  vvould  seem  to  be  of  an  albumi- 
nous nature,  and  it  is  distinguishable  from  the  cell-contents 
by  being  left  untinged  by  an  ammoniacal  solution  of  car- 
mine. In  vegetable  cells,  the  cell-wall  is  formed  of  cellulose, 
and  in  old  cells  this  is  much  thickened  by  the  deposition  of 
numerous  concentric  layers  of  woody  tissue  or  lignine  on 
its  inner  surface. 

2.   The  Cell-contents. — The  materials  comprised  within  the 


mi^lMMm^gfjltl^m^^ 


74  ELEMENTS  OF   BIOLOGY. 

cell-wall,  irrespective  of  and  not  including  the  "  nucleus," 
when  this  is  present,  are  usually  termed  the  "  cell-contents." 
'I'he  nature  of  these  materials  varies  much  in  some  respects 
of  minor  importance ;  but  it  is  probable  that  the  cell-con- 
tents are  to  be  regarded  as  essentially  of  the  nature  of  proto- 
plasmic or  bioplasmic  matter.  This,  at  any  rate,  is  the  case 
in  young,  actively-growing  cells,  and  in  these  the  cell-wall  bears 
a  small  proportion  to  the  cell-contents.  In  progress  of  growth, 
however,  the  cell-contents  seem  to  diminish  in  bulk,  owing 
to  the  conversion  of  their  outermost  layers  into  "  formed" 
material.  The  cell-contents  are  deeply  reddened  by  a  solu- 
tion of  carmine  (fig.  19),  and  generally  contain  more  or  less 
numerous  molecules  and  granules.  Upon  the  whole,  the 
cell-contents  appear  to  be  the  essential  and  most  important 
element  of  the  cell.  They  constitute  the  only  element  the 
existence  of  which  is  constant ;  and  ihey  are  the  main,  or, 
in  some  cases,  the  sole,  agent  whereby  the  vital  actions  of 
the  cell  are  carried  on. 

3.  The  Nucleus. — Very  generally,  but  by  no  means  uni- 
versally, the  cell-contents  exhibit  in  one  place  a  definite 
rounded  or  oval  body,  which  is  termed  the  "  nucleus " 
(fig.  20,  c).  This  varies  much  in  actual  structure,  'some- 
times being  vesicular,  sometimes  solid,  and  sometimes  com- 
posed of  granules.  That  the  nucleus  plays  an  important 
part  in  cell-life  cannot  be  doubted  "^  but  opinions  are  still 
divided  as  to  its  exact  functions,  some  regarding  it  as  the 
most  important  agent  in  cell-activity,  whilst  others  consider 
it  of  comparatively  small  moment.  That  it  is  composed  of 
growing  and  living  matter  is  shown  by  the  extent  to  which 
it  is  coloured  by  carmine,  and  it  seems  in  many  cases  to 
take  the  initiative  in  the  process  of  cell-multiplication.  It 
is  not  invariably  present,  however,  and  it  v/ould  not,  there- 
fore, seem  to  be  absolutely  essential  to  cells.  On  the  other 
hand,  "free"  nuclei,  which  have  been  liberated  from  cells, 
sometimes  play  a  most  important  part  in  various  vital  pro- 
cesses. 


CELLS. 


75 


Not  uncommonly,  the  nucleus  contains  in  its  interior  a 
still  more  minute  solid  body  or  particle,  which  is  known  as 
the  "  nucleolus."  The  functions,  however,  of  the  nucleolus 
are  not  known  with  any  precision,  and  it  is  often  absent. 

Cell-multiplication. — When  once  formed,  cells  not 
only  grow  and  maintain  their  existence  during  an  active 
period  of  varying  length,  but  they  have  also  the  power,  in 
many  cases,  of  producing  fresh  cells  by  a  process  of  cell- 
multiplication  or  "  cytogenesis."  The  modes  in  which  this 
is  effected  vary  in  different  cases,  but  they  may  be  reduced 
to  three  principal  forms  : — 

a.  Endogenous  Cell-multiplication. — In  this  method  new 
cells  are  produced  within  a  parent-cell  by  the  separation  of 
the  cell-contents  into  a  greater  or  less  number  of  distinct 
masses,  each  of  which  may  become  ultimately  enclosed  in  a 
proper  cell-wall  (fig. 
2i).  This  method  of 
multiplication  is  well 
seen  in  the  fecundated 
ovum,  and  it  appears 
to  commence  by  the 
cleavage  or  division 
into  two  parts  of  the 
nucleus.  The  cell  - 
contents  then  become 

aggregated  round  each  half  of  tne  nucleus  so  as  to  form  two 
cells  within  the  parent-cell.  The  nuclei  divide  again  in  a 
similar  manner,  giving  rise  to  four  cells ;  these  divide  again, 
giving  rise  to  eight  cells ;  and  so  the  process  may  go  on, 
till  there  is  formed  in  the  primitive  cell  an  indefinite  num- 
ber of  new  cells.* 


a 


Fig.  21. — Cleavage  of  the  yolk  of  the  ovum  of 
Ascaris  nigroi-enosn  (after  KOlliker). 


*  The  term  of  "endoge-  jJ"-  cell-multiplication"  was  originally  applied 
to  cases  in  which  the  cell-contents  divided  into  fresh  cells  without  any 
participation  of  the  cell-wall.  It  is  now  known,  however,  that  the  cell- 
wall  never  takes  any  part  in  the  process  of  cell-multiplication.  It  has 
been  proposed,  therel   re,  to  restrict  the  term  "endogenous"  to  that 


m 


76 


ELEMENTS  OF  BIOLOGY. 


b.  GemmiparoHs  Cell-multiplicatioti. — In  this  process  new 
cells  arc  formed  by  little  buds  or  outward  processes,  which 
are  thrown  out  by  a  parent-cell  (fig.  22).     Each  little  bud 

appears  to  consist  of  the 
©      gini    iiiiffc     ^,  ;         ^^        living  matter  or  bioplasm 

contained  within  the  cell; 
and  it  either  thrusts  out 
a  portion  of  the  cell-wall, 
or,  as  stated  by  Beale, 
gains  access  to  the  ex- 
terior by  minute  pores  in 
the  limiting  membrane  of 
the  cell.  The  cells  thus 
produced  may  remain  at- 
tached to  the  parent-cell, 
and  may  repeat  the  pro- 
cess of  gemmation ;  or  they  may  become  detached  to  lead 
an  •independent  existence. 

c.  Fissiparous  Cell-7niiHiplication. — In  this  process  a  parent- 
cell  divides  by  cleavage  or  fission  into  two  or  four  parts, 
each  of  which  becomes  a  perfect  and  independent  cell.  This 
process  is  by  no  means  so  important  as  the  two  preceding, 
and  it  is  doubtful  if  it  exists  at  all,  except  as  a  modification 
of  endogenous  cell-multiplication,  if  we  employ  this  term  in 
the  wide  sense  in  which  it  is  used  above. 

form  of  cytogenesis,  in  which  new  cells  are  produced  in  a  parent-cell 
round  independent  nuclei,  without  the  nucleus  of  the  latter  dividing  or 
taking  any  share  in  the  process. 


Fig.  22. — Cells  of  the  Yeast-plant,  producing 
frosh  cells  by  a  process  of  gemination. 
Magnified  2800  diameters.    (After  Uealo.) 


!|M. 


1-1 

■\ ! 
"!   i. 


^CHAPTER    VIII. 


PHYSIOLOGICAL   FUNCTIONS   OF  ANIMALS   AND   PLANTS. 

As  has  been  before  remarked,  all  the  vital  processes  of  ani- 
mals and  plants  may  be  considered  under  three  heads :  i. 
Functions  of  Nutrition^  comprising  all  those  functions  where- 
by the  individual  organism  lives,  grows,  and  maintains  its 
existence  against  all  .he  hostile  forces  constantly  at  work 
upon  it.  2.  Functions  of  Reproduction^  comprising  those 
functions  whereby  the  perpetuation  of  the  species  is  secured, 
while  the  individual  perishes.  3.  Functions  of  Relation^  com- 
prising all  those  functions,  such  as  sensation  and  locomo- 
tion, whereby  the  organism  is  brought  into  relation  with  the 
outer  world,  and  the  outer  world  in  turn  reacts  upon  the 
organism. 

In  plants  the  functions  of  relation  are  reduced  to  their 
minimum,  and  hence  these  functions  are  often  spoken  of  as 
the  Animal  functions  ;  whilst  the  functions  of  nutrition  and 
reproduction,  as  being  ronunon  to  all  organisms,  are 
grouped  together  under  the  nartie  of  the  Organic  or  Vegeta- 
tive functions.  Plants,  however,  are  by  no  means  wholly 
destitute  of  the  functions  of  relation ;  and,  curiously  enough, 
these  functions  are  most  developed,  or,  at  any  rate,  most 
conspi'  tious,  in  some  of  the  lowest  member^  of  t'  "^  vegetable 
kingdom,  h  have  on  this  account  been  mistaken  for 

animals. 


78  ELEMENTS  OF  BIOLOGY. 

In  plants  we  observe  the  same  specialisation  of  functions 
that  we  have  formerly  seen  in  animals,  in  ascending  from  the 
lowest  forms  to  the  highest ;  but,  as  in  animals  also,  there  is 
an  apparent  reversal  of  this  law  in  some  cases  as  far  as  the 
functions  of  reproduction  are  concerned.  The  processes, 
namely,  by  which  a  young  Exogen  is  produced,  are  appa- 
rently less  complex  than  those  by  which  many  of  the  Cryp- 
togams are  perpetuated,  just  as  the  reproduction  of  a  Verte- 
brate animal  is  in  one  way  a  simpler  matfer  than  that  of  a 
Hydroid  Zoophyte.  In  all  these  cases,  as  we  shall  see,  the 
essential  part  of  the  process  consists  in  the  bringing  together 
of  a  germ-cell  or  ovum  and  a  sperm-cell  or  spermatozoid ; 
and  so  far  as  the  process  of  bringing  together  is  concerned, 
the  complexity  is  certainly  on  the  side  of  the  lower  form. 
It  may  be  said,  also,  that  there  are  no  essential  or  funda- 
mental characters  by  which  the  ova  and  sperm-cells  of  the 
higher  form  can  be  distinguished  from  those  of  the  lower. 
This  is  one  of  the  cases,  however,  in  which  simplicity  of 
anatomical  structure  must  not  be  confounded  with  simplicity 
of  function.  The  sperm-cells  of  a  Vertebrate  animal  may 
not  to  our  eyes  seem  very  different  from  those  of  one  of  the 
Algae ;  but  unquestionably  this  can  only  be  because  our 
means  of  observation  are  not  of  such  a  nature  as  to  disclose 
to  us  the  subtle  but  immense  differences  which  must  of 
necessity  exist.  In  all  cases,  also,  it  is  to  be  borne  in  mind 
that  the  organs  by  which  the  generative  elements  are  elabo- 
rated are  of  a  more  complex  description  in  the  higher 
organism. 

In  some  of  the  lower  plants,  such  as  the  Yeast-plant 
(fig.  19),  all  the  great  physiological  functions  are  carried  on 
by  single  cells,  without  the  presence  of  any  differentiatp.d 
internal  organs.  In  such  simple  plants,  also,  so  far  as  our 
means  of  observation  allow  us  to  judge,  all  the  peculiarities 
which  distinguish  plants  physiologically  from  animals  are  as 
strongly  pronounced  as  they  are  in  the  highest  vegetables. 
In  such  forms,  then,  as  the  yeast-plant,  we  have  a  single  cell 


I    i 


VITAL   FUNCTIONS.  79 

discharging  all  the  vital  functions ;  but  it  is  noticeable  that 
this  is  the  lowest  step  in  the  ladder  of  life  to  which  any  veget- 
able descends.  A  cell  is  an  organised  structure,  and  no 
adult  plants  appear  to  possess  the  power  of  discharging  all 
the  vital  functions  with  a  less  amount  of  vital  machinery 
than  this.  In  animals,  however,  as  already  remarked,  we 
meet  with  forms  which,  from  a  purely  morphological  point 
of  view,  are  certainly  below  the  lowest  plants.  The  Monera 
of  Haeckel,  and  the  Foratnini/era^  discharge  their  vital  func- 
tions wholly  through  the  medium  of  stnictureless  albumi- 
nous matter  or  **  bioplasm,"  which  is  destitute  of  a  proper 
wall,  and  never  has  definite  structures  developed  in  it.  It 
may  be  that  the  matter  of  life  is  in  these  creatures  of  a 
higher  grade  than  it  is  in  the  lower  plants,  but  there  is  no 
direct  evidence  which  would  support  this  view.  It  is  cer- 
tain, however,  that  there  is  a  radical  difference  between  the 
living  matter  of  an  animal,  such  as  one  of  the  Foraniinifera^ 
and  a  plant,  such  as  a  cell  of  yeast,  since  both  discharge 
functions  of  a  radically  different  nature ;  but  tHere  is  no 
ground  for  supposing  that  this  difference  is  one  of  chemical 
composition  or  physical  character.  We  can  also  readily 
see  that  the  vital  processes  of  one  of  the  Foratninifcra  differ 
so  far  from  those  of  a  plant,  especially  as  regards  the  in- 
gestion of  food,  that  the  structure  of  the  vegetable  cell 
would  be  obviously  unsuited  to  the  higher  organism. 

In  the  higher  animals  and  plants  we  are  presented  with 
structures  which  may  be  regarded  as  essentially  aggregates 
of  cells ;  and  there  is  now  a  "  physiological  division  of 
labour,"  some  of  the  cells  being  concerned  with  the  nutri- 
tion of  the  organism,  whilst  others  are  set  apart  and  dedi- 
cated to  the  function  of  reproduction.  Every  cell  in  such 
an  aggregate  leads  a  life  which  in  a  certain  limited  sense 
may  be  said  to  be  independent,  and  each  discharges  its  own 
function  in  the  general  economy.  Each  ceil  has  a  period 
of  development,  growth,  and  active  life,  and  each  ultimately 
perishes ;  the  life  of  the  organism  not  only  uot  depending 


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80  ELEMENTS  OF  BIOLOGY. 

upon  the  life  of  its  elemental  factors,  but  actually  being 
kept  up  by  their  constant  destruction  and  as  constant  re- 
newal. Only  in  a  very  limited  sense,  therefore,  can  we  say 
that  the  life  of  the  organism  is  the  sum  total  of  the  lives  of 
these  structural  units. 

Lastly,  in  plants  as  in  animals,  the  vital  processes  are 
carried  on  by  forces  which  we  cannot  as  yet  refer  'cnown 
chemical  and  physical  forces,  and  which,  therefore,  .  e  are, 
in  the  meanwhile,  compelled  to  speak  of  as  "  vital."  In 
the  case  of  plants,  for  example,  it  is  quite  true  that  certain 
known  chemical  and  physic.  '  ^orces  are  concerned  in  their 
vital  processes,  and  are,  indeeu,  absolutely  necessary  for 
their  due  performance.  Thus  it  is  absolutely  certain  that 
no  plant  can  convert  inorganic  matter  into  organic  com- 
po'j/>ds,  or,  in  other  words,  can  digest,  unless  it  be  supplied 
with  solar  light;  whilst  the  solar  heat  is  equally  essentiat 
to  the  performance  of  other  of  its  vital  processes.  On  this 
subject  Dr  Carpenter  expresses  himself  as  follows  : — 

"  Planft;  form  thuse  organic  compounds  at  the  expense  of 
which  animal  life  (as  well  as  their  own)  is  sustained,  by 
the  decomposition  of  carbonic  acid,  water,  and  ammonia ; 
and  the  light,  by  whose  agency  alone  these  compounds  can 
be  generated,  may.  be  considered  as  metamorphosed  into 
the  chemico-viial  affinity  by  which  their  components  are  held 
together.  The  heat  which  plants  receive,  acting  through 
their  organised  structures  as  vital  force,  serves  to  augment 
these  structures  to  an  almost  unlimited  extent,  and  thus  to 
supply  new  instruments  for  the  agency  of  light  and  for  the 
production  of  organic  compounds.  Supposing  no  animals 
existed  to  consume  these  organic  compounds,  they  would 
all  be  restored  to  the  unorganised  condition  by  spontaneous 
decay,  which  would  reproduce  carbonic  acid,  water,  and 
ammonia,  from  which  they  were  generated.  In  this  decay, 
however  slow,  the  same  amount  of  heat  would  be  given  off 
as  in  the  more  rapid  processes  of  combustion  -,  and  the 
faint  luminosity  which  has  been  observed  in  some  vegetable 


VITAL  FUNCTIONS.  8 1 

substances  in  a  state  of  eremacaiisis  "  (slow  decay)  "  makes 
it  probable  that  the  same  is  true  of  light."  i   ,: 

In  considering  the  doctrine  here  laid  down  as  to  the 
identity  of  the  chemico-vital  forces  concerned  in  the  nutri- 
tive processes  of  plants  with  the  sun-light  and  sun-heat, 
the  student  must  guard  himself  against  confounding  the 
necessary  conditions  of  a  phenomenon  with  its  cause.  The 
chemical  and  calorific  rays  of  the  sun  are  doubtless  essen- 
tial to  the  performance  by  plants  of  their  vital  functions ; 
but  it  does  not  follow  that  they  are  the  only  forces  resi- 
dent in  the  vegetable  organism,  or,  indeed,  that  they  are 
the  most  important  ones.  The  true  difficulty  of  the  pro- 
blem lies  in  this  very  transformation  of  purely  physical 
energy  into  chemico-vital  forces.  How  do  plants  convert 
sun-light  into  the  chemical  affinity  by  which  they  are  enabled 
to  raise  certain  stable  inorganic  materials  to  the  height  of 
unstable  organic  compounds  ?  How,  and  in  virtue  of  what, 
do  plants  convert  sun-heat  into  the  vital  force  by  which 
they  can  increase  their  organised  structures  "  to  an  almost 
unlimited  extent?"  Here  lies  the  true  problem,  and  it  is 
one  from  the  solution  of  which  we  are  very  far  as  yet.  It 
is  no  real  explanation  to  say  that  the  mechanism  or  the 
material  of  the  plant  is  such  as  to  produce  this  change,  just 
as  when  we  transmit  heat  through  a  given  apparatus  and  it 
becomes  electricity,  or  through  another  and  it  is  converted 
into  light.  No  one  doubts  the  possibility,  and  truly  the 
daily  occurrence,  of  such  transformations,  but  this  affords 
no  true  explanation  in  the  case  of  the  plant.  In  the  first 
place,  this  explanation  begs  the  very  question  at  issue,  for 
it  assumes  what  cannot  be  proved — namely,  that  the  change 
is  effected  by  the  "physical  basis"  of  the  plant,  instead  of 
by  some  special  power  residing  in  the  organism.  It  as- 
sumes, also,  that  all  the  forces  expended  by  the  plant  in 
its  vital  work  are  the  exact  equivalent  of  the  solar  heat  and 
light  which  it  receives — an  assumption  which  may  be  highly 
probable,  but  is  nevertheless  incapable  of  proof.     In  the 


82  ELEMENTS  OF  BIOLOGY. 

second  place,  it  leaves  us  wholly  in  the  dark  as  to  why  the 
albuminoid  or  other  matter  of  a  Protophyte  should  have 
this  power,  whilst  the  very  similar  living  matter  of  a  Pro- 
tozoon  should  be  wholly  without  it.  Lastly,  this  explana- 
tion could,  at  best,  but  apply  to  the  nutritive  processes  of  a 
plant,  and  would  not  by  anv  means  wholly  elucidate  these. 
The  phenomena  of  reproduction  cannot  be  explained  by  the 
action  of  any  known  chemical  or  physical  forces ;  though 
such  forces  are  necessary  -conditions  for  these,  just  as  they 
are  for  the  phenomena  of  nutrition.  To  say  that  a  plant 
could  not  perpetuate  its  species  unless  it  were  supplied  with 
solar  light  and  heat,  would  be  true  enough ;  but,  after  all, 
it  would  amount  to  no  more  than  saying  that  the  plant 
would  not  be  alive  at  all  except  under  these  conditions. 

ID  the  present  state  of  our  knowledge,  therefore,  we 
must  conclude  that  we  cannot  refer  all  the  forces  which  we 
see  at  work  in  the  vegetable  organism  to  known  chemical 
or  physical  forces.  Even  those  physical  and  chemical 
forces  which  we  knoiv  to  be  present  in  the  plant,  act  in  a 
manner  different  to  what  they  would  do  in  any  collocation 
of  dead  matter,  or  in  any  animal ;  whilst  there  are  super- 
added other  phenomena  which  we  cannot  at  present  ex- 
plain, and  which  we  cannot  therefore  refuse  to  call  "vital." 
Admitting  that  the  conversion  by  the  plant  of  inorganic 
materials  into  organic  compounds  is  a  purely  chemical 
operation,  there  would  still  remain  the  fact,  as  pointed  out 
by  Dr  Beale,  that  it  is  ii  chemical  process  differing  alto- 
gether from  any  and  all  processes  which  we  can  imitate  in 
the  laboratory.  Thus  the  most  degraded  of  the  plants 
effgcts  "  silently  and  in  a  moment y  without  apparatus,  with 
little  loss  of  material,  at  a  temperature  jf  60°  or  lower, 
changes  in  matter  some  of  which  can  be  imitated  in  the 
laboratory  in  the  course  of  days  or  weeks  by  the  aid  of  a 
highly-skilled  chemist,  furnished  with  complex  apparatus 
and  the  means  of  producing  a  very  high  temperature  and 
intense  chemical  action,  and  with  an  enormous  waste  of 


VITAL  FUNCTIONS.  $$ 

material."  It  is  obvious,  then,  that  there  is  something  in 
the  so-called  "  chemical "  processes  of  the  plant  over  and 
above  ordinary  chemical  action  as  known  to  us ;  and  that 
something,  in  our  ignorance  of  its  nature,  we  may  still  call 
"vital,"  even  though  we  beheve  that  it  will  ultimately  be 
shown  to  be  nothing  more  than  a  modification  of  some 
physical  force. 


,* 


CHAPTER    IX. 

GENERAL  PHENOMENA   OF   NUTRITION. 

Nutrition  is  the  name  applied  to  all  those  processes  by 
which  the  organism  maintains  its  existence  as  an  individual. 
In  the  more  degraded  forms  of  life  nutrition  is  a  com- 
paratively simple  process ;  but,  in  accordance  with  the  law 
of  the  specialisation  of  functions,  it  becomes  a  very  com- 
plicated matter  in  the  higher  forms.  It  is  unnecessary  to 
say  that  it  is  impossible  here  to  examine  the  different  modes 
in  which  nutrition  is  effected  in  different  organisms ;  and  all 
thai  can  be  atten»pted  will  be  to  give  a  very  brief  and 
general  sketch  of  the  process  as  a  whole. 

Every  vital  act  in  every  organism  appears  to  be  effected 
at  the  expense  of  the  structure  by  which  the  act  is  performed. 
Whenever  a  muscle  contracts — thus  performing  its  proper 
function — a  portion  of  its  substance  is  destroyed ;  and  this 
holds  good  of  every  tissue  and  of  every  function.  It  follows 
from  this  that  life  is  accompanied  by  constant  but  partial 
death  of  the  matter  of  life ;  and  the  loore  actively  and 
perfectly  any  organism  exercises  its  vital  functions,  the  more 
rapidly  does  it  destroy  the  material  basis  by  which  the 
vitality  is  manifested.  It  follows,  also,  from  this,  that  the 
constant  loss  of  substance  caused  by  the  exercise  of  vital 
acts  must  be  as  constantly  repaired,  if  the  organism  is  to 
maintain  its  integrity.     This  can  only  be  effected  by  the  con- 


GENERAL   PHENOMENA  OF   NUTRITION. 


85 


stant  formation  of  fresh  tissue  to  take  the  place  of  that  which 
has  been  destroyed  by  use  j  and  in  this  essentially  consists 
the  nutrition  of  an  organism  in  its  adult  condition.  Every 
organism,  then,  is  compelled  to  be  incessantly  manufactur- 
ing fresh  matter  fit  to  replace  the  losses  caused  by  vital 
action  ;  and  the  power  by  which  this  is  effected  is  known  by 
the  general  name  of  assimilation.  With  the  exception, 
namely,  of  parasites  living  on  the  already  elaborated  juices 
of  their  hosts,  no  organism  takes  as  food  materials  which 
can  be  built  up  directly  and  without  change  into  new  tissue. 
On  the  contrary,  the  materials  taken  as  food  have  to 
imdergo  certain  changes  before  they  can  be  employed  in 
repairing  loss — they  have  to  be  "assimilated"  or  made  like 
to  the  tissue  which  they  are  to  replace. 

The  power  of  assimilation  is  one  of  the  most  remarkable 
of  the  properties  of  living  matter;  and  it  is  one  which 
resides,  not  in  the  organism  as  a  whole,  but  in  each  in- 
dividual portion  and  every  separate  tissue  of  the  organism. 
However  simple  may  be  the  being  with  which  we  have  to 
deal,  and  even  if  there  be  no  special  alimentary  apparatus, 
the  general  result  of  the  digestive  process  is  the  production 
of  a  common  nutritive  fluid,  which  contains  certain  organic 
compounds  manufactured  out  of  the  food  during  the  process 
of  digestion.  In  the  case  of  the  higher  animals,  this  com- 
mon nutrient  fluid  is  called  the  bloody  and  there  is  usually  a 
special  organ  or  "  heart,"  by  which  it  is  propelled  through 
special  tubes,  or  "  blood-vessels,"  to  every  organ  and  every 
tissue  in  the  body.  Many  of  the  lower  animals  are  destitute 
of  any  such  special  apparatus ;  but  in  all  cases  the  nutrient 
fluid  which  is  the  result  of  the  digestive  process,  and  which 
corresponds  with  the  blood  of  the  higher  forms,  is  distributed 
to  all  parts  of  the  organism.  The  blood,  however,  or  the 
nutrient  fluid  which  takes  its  place,  is  simply  a  solution  of 
certain  organic  compounds,  and  the  assimilation  of  these 
compounds  is  efi"ected  in  the  tissues  themselves,  the  part 

played  by  the  blood  being  the  merely  passive  one  of  serving 

5  •  ■ 


86  ELEMENTS  OF  BIOLOGY. 

as  a  vehicle.  Every  tissue  takes  from  the  common  store- 
house of  the  blood  just  those  materials  which  it  requires, 
and  builds  these  up  into  matter  similar  to  itself.  The 
muscles  take  from  the  blood  the  substances  necessary  to 
form  muscle,  the  bones  take  the  materials  required  for  the 
production  of  osseous  tissue,  and  so  on.  Every  tissue, 
therefore,  possesses  the  power  of  replacing  the  particles 
destroyed  by  its  functional  activity,  by  manufacturing,  so  to 
speak,  particles  equal  in  number  and  similar  in  character  to 
those  which  have  died.  Hence,  a  tissue  may  remain  for  an 
indefinite  period  apparently  unchanged,  though  constantly 
active  and  constantly  suffering  loss  of  substance.  Hence, 
also,  as  every  tissue  has  the  power  of  thus  maintaining  its 
integrity  by  the  assimilation  of  new  matter,  the  entire  or- 
ganism may  remain  unaltered  in  appearance  throughout  a 
long  period  of  active  life,  though  actually  the  seat  of  inces- 
sant loss  and  equally  incessant  repair.  And,  in  those  beings 
in  which  the  body  is  composed  of  a  uniform  substance  not 
exhibiting  any  differentiation  into  distinct  organs,  the  as- 
similation of  the  individual  particles  of  the  body  becomes 
undistinguishably  merged  in  assimilation  by  the  organism  as 
a  whole. 

By  means  of  the  pu»ver  of  assimilation,  as  above  described, 
every  organism  possesses  the  power  of  maintaining  a 
certain  average  condition  during  a  longer  or  shorter  period 
of  active  life,  its  losses  being  exactly  balanced  by  its  gains. 
There  is,  however,  a  fundamental  difference  between  all 
animals  and  the  majority  of  plants  as  to  the  powers  which 
they  possess  of  preparing  nutrient  matter  to  be  subsequently 
assimilated.  All  animals,  without  a  known  exception,  re- 
quire to  be  supplied  with  ready-made  organic  compounds 
for  their  food.  The  food  need  not  necessarily  contain  the 
exact  organic  compounds  which  the  animal  requires  to  build 
up  its  tissues.  Indeed,  in  the  great  majority  of  cases,  if 
not  in  all,  the  organic  compounds  of  the  food  have  to 
undergo  certain  changes  before  they  can  be  actually  cm- 


GENERAL  PHENOMENA  OF   NUTRITION. 


87 


ployetl  ^y  the  animal  to  repair  its  losses.  Still,  no  animal 
can  live  upon  inorganic  matter  alone,  and  the  food  must 
therefore  have  been  derived  from  a  pre-existent  organism. 
A  few  plants  agree  with  animals  in  this  respect,  and  may 
therefore  be  looked  upon  as  animals  so  far  as  their  food  is 
concerned. 

The  great  majority  of  plants,  on  the  other  hand,  are 
endowed  with  the  power  of  converting  inorganic  materials 
into  organic  compounds,  and  they  thus  differ  altogether  from 
animals.  The  formative  power  of  plants  in  this  respect  is, 
however,  limited  by  very  definite  bounds.  The  tissues  of 
plants  consist  mainly  of  carbon,  hydrogen,  oxygen,  and 
nitrogen ;  but  plants  cannot  avail  themselves  of  these 
elements  as  such.  Thus,  nitrogen  is  largely  present  in  the 
atmosphere  by  which  terrestrial  plants  are  surrounded,  but 
plants  do  not  derive  their  supply  of  this  element  from  this 
source.  The  nitrogen  of  plants  is,  on  '.he  contrary,  obtained 
by  them  from  ammonia,  which  they  absorb  from  the  soil. 
Similarly,  the  carbon  of  plants  is  obtained  from  the  carbonic 
acid  which  is  contained  in  the  atmosphere,  or  is  dissolved 
in  the  water  taken  up  by  the  roots. 

Hitherto  we  have  been  dealing  with  an  adult  organism, 
and  we  have  seen  how  the  result  of  nutrition  to  maintain 
the  living  body  in  a  practically  unchanged  condition,  by  the 
continual  formation  of  fresh  matter  to  take  the  place  of  that 
which  has  been  destroyed  by  vital  work.  In  this  process 
it  is  obvious  that  an  average  condition  of  the  organism  can 
only  be  maintained  so  long  as  the  production  of  new  matter 
is  more  or  less  exactly  equivalent  to  the  destruction  of  old 
matter.  The  processes  of  repair  and  waste  must  go  hand 
in  hand,  and  neither  must  exceed  the  other  for  any  length 
of  time.  The  formation  of  new  matter  may,  however,  fall 
short  of  the  destruction  of  old  matter,  or  it  may  exceed  it ; 
and  in  either  case  we  have  a  fresh  series  of  phenomena  to 
observe. 

When  the  waste  of  the  organism  caused  by  the  discharge 


88  ELEMENTS  OF  BIOLOGY. 

* 

of  its  vital  functions  no  longer  is  repaired  by  a  concurrent 
and  equivalent  process  of  repair,  it  is  clear  that  life  cannot 
be  maintained  for  any  length  of  time.  If  we  are  dealing 
with  a  single  definite  ])art  or  organ,  such  as  an  individual 
muscle,  the  result  of  this  state  of  things  is  a  progressive 
**  atrophy."  If  the  organ  is  not  one  necessary  to  life  the 
organism  may  survive,  but  the  organ  affected  becomes  ulti- 
mately unable  to  discharge  its  vital  functions,  and  practically 
dies,  so  far  as  the  general  econoniy  is  concerned.  If  a  vital 
organ  is  thus  affected,  or  if  the  nutritive  failure  extends  to 
the  entire  organism,  the  final  result,  however  long  delayed, 
is  necessarily  death.  When  an  animal  dies  simply  of  "  old 
age,"  it  is  probably  in  consequence  of  this  failure  of  nutri- 
tion to  supplement  the  incessant  losses  caused  Ly  living. 
It  is  to  be  remembered,  however,  that  we  have  undoubtedly 
to  deal  here  with  a  deeper  law,  not  connected  apparently 
with  the  above.  It  might  be  thought  that  there  is  no  reason 
why  any  animal  should  not  live  for  an  indefinite  period,  pro- 
vided it  could  but  maintain  the  standard  of  nutrition,  so 
thai  the  losses  of  life  should  never  for  long  exceed  the 
powers  of  repair.  1  his,  however,  does  not  seem  to  be 
even  theoretically  true.  It  seems,  on  the  contrary,  that 
there  is  for  every  species  of  animal  a  certain  comparatively 
definite  limit  beyond  which  its  life  can  not  be  prolonged. 
It  appears  to  be  like  a  machine,  "made  "  to  run  a  certain 
time,  but  certain  to  break  down  after  reaching  a  given  limit. 
Some  individuals  of  the  species  do  not  reach  this  limit ; 
other  individuals  may  exceed  it ;  but  the  limit  remains  for 
the  species  as  an  "  average  period  of  life,"  which  a  few  over- 
pass, whilst  the  majority  never  reach  it. 

If,  on  the  other  hand,  the  process  of  repair  exceeds  that 
of  waste — if  new  material  is  added  faster  than  old  material 
is  destroyed — then  we  have  the  state  of  things  which  is  pro- 
perly termed  growth.  Growth  may  be  of  the  organism  as  a 
whole,  or  of  any  particular  part  or  organ  ;  and  in  either  case 
it  consists  simply  in  the  addition  of  matter  similar  in  kind 


DEVELOPMENT.  9^ 

to  that  already  existing,  but  exceeding  in  quantity  that  which 
is  being  destroyed.  In  the  process  of  growth,  therefore,  in 
the  strict  sense  of  this  term,  there  cannot  occur  any  change 
in  the  actual  fonn  or  composition  of  the  growing  body.  The 
part,  or  the  organism  as  a  whole,  increases  in  density  or 
size  by  the  addition  of  particles  similar  to  those  of  which  it 
already  consists ;  but  no  change  takes  place  in  its  essential 
characters,  or  in  the  functions  which  it  is  capable  of  dis- 
charging. 

DEVELOPMENT. 

We  have  in  the  preceding  been  considering  the  processes 
by  which  an  organism,  or  any  part  of  an  organism,  is  enabled 
to  grow,  or,  when  fully  grown,  is  enabled  to  maintain  itself 
for  a  longer  or  shorter  period  in  a  stationary  condition.  We 
have  now  very  briefly  to  consider  the  processes  by  which 
any  organism  becomes  what  we  see  it  to  be,  or  by  which  a 
given  organ  is  for  the  first  time  formed  and  brought  to 
maturity.  To  all  these  processes  the  term  "  Development  '* 
is  applied,  but  we  are  here  only  concerned  with  those  which 
relate  to  the  organism  as  a  whole. 

From  this  point  of  view  the  term  Development  includes  all 
those  changes  which  a  geim  undergoes  before  it  assumes 
the  characters  of  the  perfect  individual ;  and  the  chief  dif- 
ferences which  are  observed  in  the  process  as  it  occurs  in 
different  animals  consist  simply  in  the  extent  to  which  these 
changes  are  external  and  visible,  or  are  more  or  less  com- 
pletely concealed  from  view.  For  these  differences  the 
terms  "  transformation "  and  "  metamorphosis  "  are  em- 
ployed ;  but  they  must  be  regarded  as  essentially  nothing 
more  than  variat/ons  of  development. 

Transformation  is  the  term  employed  by  Quatrefages  to 
designate  "  the  series  of  changes  which  every  germ  under- 
goes in  reaching  the  embryonic  condition ;  those  which  we 
observe  in  every  creature  still  within  the  t%g  ;  those,  finally, 


I 


I    \ 


90 


ELEMENTS  or   IIIOI.OGY. 


I' 


which  the  species  born  in  an  impcrfcc  lly-dcvclopcd  state 
present  in  the  course  of  their  external  life." 

Metamorphosis  is  defined  by  the  sam°  author  as  including 
the  alterations  which  are  "undergone  after  exclusion  from 
the  Q^'g^  and  which  alter  extensively  the  general  forn^  and 
mode  of  life  of  the  individual." 

'I'hough  by  no  means  faultless,  these  terms  are  sufficiently 
convenient  in  practice,  if  it  be  remembered  that  they  are 
merely  modifications  of  development,  and  express  differen- 
ces of  a  degree  and  not  of  kind.  An  insect,  such  as  a  But- 
terfly, furnishes  js  with  the  most  striking  illustration  of  what 
is  meant  by  these  terms.  All  the  changes  which  are  undergone 
by  a  Butterfly  in  passing  from  the  fecundated  ovum  to  the 
condition  of  an  "  imago  "  or  perfect  insect,  constitute  its 
development.  The  egg  laid  by  a  Butterfly  undergoes  a  series 
of  changes  which  eventuate  in  its  giving  bi''Mi  to  a  caterpillar 
or  "  larva  "  (fig.  23.  a),  these  preliminary  changes  constitut- 
ive; its  transformation.     The  caterpillar  is  totally  unlike  the 


Fig.  23. — Large  White  Cabbage  Butterfly  (Pontia  brassicip).    n  Larva  or  Caterpillar; 
b  Pupa  or  Chrysalis ;  c  Imugo  or  perfect  Insect. 

adult  insect  in  appearance,  and  possesses  organs  which  adapt 
it  to  a  totally  different  mode  of  life.     It  grows  rapidly  in 


T)KV1J.<)1'MKNT.  91 

size,  but,  though  it  repeatedly  changes  its  skin,  it  retains  its 
characters  for  a  longer  or  shorter  period.  It  then  ceases  to 
cat,  becomes  enveloped  in  a  chitinous  skin,  and  loses  all  its 
former  powers  of  locomotion.  It  now  constitutes  what  is 
known  as  the  "chrysalis"  or  "pupa"  (fig.  23,  l>).  In  this 
quiescent,  moiionless,  and  apparently  dead  condition  it 
remains  for  a  longer  or  shorter  time,  during  which  develop- 
mental changes  are  going  on  rapidly  in  its  interior.  Finally, 
the  chrysalis  nirtures,  and  there  escapes  from  it  the  perfect 
winged  insect  or  "imago"  (fig.  23,  c).  To  these  changes 
the  term  metamorphosis  is  rightly  applied.  These  changes, 
however,  do  not  diffci  in  kind  from  the  changes  undergone 
by  a  Mammal ;  tlie  difference  being  that  in  the  case  of  a 
Mammal  the  ovum  is  retained  within  the  body  of  the  parent, 
where  it  undergoes  the  necessary  developmental  changes, 
so  that  at  birth  it  has  litde  to  do  but  grow,  in  order  to  be 
converted  into  the  adult  animal. 

From  these  considerations  we  arrive  at  the  generalisation 
laid  down  by  Quatrefages  :  "  Those  creatures  whose  ova 
— owing  to  an  insufficient  supply  of  nutritious  contents,  and 
an  incapacity  on  the  part  of  the  mother  to  provide  for  their 
complete  development  within  her  own  substance — are  ra- 
pidly hatched,  give  birth  to  imperfect  offspring,  which,  in 
proceeding  to  their  definitive  characters,  undergo  several 
alterations  in  structure  and  form,  known  as  metamorphoses." 

When  th*"  young  organism,  therefore,  is  thrown  upon  the 
world  at  a  very  early  period  of  its  development,  it  generally 
differs  much  from  the  adult  in  its  external  characters,  and 
its  mode  of  life  is  mostly  quite  different  to  that  of  the  latter. 
As  a  result  of  this,  it  commonly  happens  that  the  young 
animal  possesses  some  of  the  structures  of  the  adult  in  a 
very  much  modified  form,  whilst  it  may  possess  others 
which  are  of  a  merely  provisional  nature,  and  are  altogether 
wanting  in  the  fully-grown  organism.  Thus  the  caterpillar 
has  to  feed  upon  hard  substances,  whilst  the  butterfly  lives 
upon  vegetable  juices.    The  caterpillar,  therefore,  is  fur- 


92  ELEMENTS  OF  BIOLOGY. 

nishcd  with  masticatory  organs  adapted  for  the  division  ol 
leaves,  and  the  like.  The  parts  of  the  mouth  in  the  butter- 
fly, on  the  other  hand,  whilst  morphologically  identical  with 
those  of  the  larva,  are  so  modified  that  they  form  a  tubular 
organ,  fitted  for  the  suction  of  fluids,  whilst  the  biting  jaws 
of  the  caterpillar  are  aborted.  The  caterpillar,  again,  carries 
three  pairs  of  legs  in  ihe  front  part  of  its  body  (fig.  23,  a), 
which  correspond  with,  and  are  ultimately  converted  into,  the 
three  pairs  of  legs  possessed  by  the  adult  insect.  The  cater- 
pillar, however,  has  an  additional  series  of  locomotive  pro- 
cesses developed  upon  some  of  the  hinder  segments  of  the 
body  (fig.  23,  a),  which  processes  are  merely  of  a  provisional 
nature,  and  arc  not  present  in  the  adult  even  in  a  rudimen- 
tary form. 

In  some  cases,  however,  not  only  does  the  young  form 
exhibit  provisional  structures,  but  there  is  what  may  be 
called  a  "  provisional  larva,"  out  of  a  portion  of  which,  and 
only  a  portion,  the  adult  animal  is  developed.  Thus,  in  the 
sea-urchins  the  egg  gives  rise  to  an  actively  locomotive 
larva,  which  is  furnished  with  a  mouth  and  alimentary  canal 
jf  its  own,  and  leads  a  completely  independent  existence. 
After  a  while,  however,  there  is  formed  upon  one  side  of 
the  stomach  of  the  larva  a  mass  of  growing  material,  which 
appropriates  the  stomach,  and  is  gradually  developed  into  a 
young  sea-urchin.  Only  the  stomach,  however,  of  the  ori- 
ginal "provisional  larva"  is  thus  retained  to  form  part  of 
the  adult  organism ;  and  the  remainder  of  this  temporary 
fonn,  having  served  its  purpose,  is  either  absorbed,  or  is 
cast  off  as  useless. 

There  is  one  respect,  however,  in  which  the  adult  animal 
is  always  the  superior  of  the  young  form,  or  at  any  rate 
almost  always ;  and  that  is  in  its  possession  of  generative 
organs,  and  the  pow-er  thereby  conferred  on  it  of  producing 
fresh  individuals  by  a  true  sexual  process.  Cases  are  not 
unknown  in  which  young  and  immature  forms  can  produce 
fresh  beings  like  themselves,  but  this  is,  in  the  great  majority 


DEVELOPMENT. 


93 


of  cases,  by  non-sexual  methods  of  reproduction,  which  will 
be  subsequently  pointed  out.  The  incapacity  for  sexual 
procreation  displayed  by  young  animals  is  in  accordance 
with  an  important  and  well-established  law,  the  exposition 
of  which  we  owe  to  Dr  W.  B.  Carpenter,  that  the  process  of 
generation  is  one  opposed  to  that  of  nutrition,  and,  a  fort  tori , 
hostile  to  growth  and  development.  The  nutritive  processes 
of  the  young  animal  are  much  more  active  than  those  of  the 
adult,  and  so  long  as  this  remains  the  case,  the  generative 
functions  remain  in  abeyance.  It  is  not  till  the  organism 
has  reached  the  point  of  nutritional  equilibrium,  that  it  be- 
corn?is  capable  of  exercising  the  function  of  reproduction  in 
its  highest  and  most  genuine  phase.  ' 

Von  Baer's  Law  of  Development. — As  the  study  of 
living  beings  in  their  adult  condition  shows  us  that  the  dif- 
ferences between  those  which  are  constructed  upon  the  same 
morphological  type  depend  upon  the  degree  to  which  special- 
isation of  function  is  carried,  so  the  study  of  development 
teaches  us  that  the  changes  undergone  by  any  animal  in 
passing  from  the  embryonic  tc  the  mature  condition  are  due 
to  the  same  cause.  All  the  members  of  any  given  sub-king- 
dom, when  examined  in  their  earliest  embryonic  condition,  are 
found  to  present  the  same  fundamental  characters.  As  de- 
velopment proceeds,  however,  they  diverge  from  one  another 
with  greater  or  less  rapidity,  until  the  adults  ultima'.ely  be- 
come more  or  less  different,  the  range  of  possible  modifica- 
tion being  apparently  almost  illimitable.  The  differences 
are  due  to  the  different  degrees  of  specialisation  of  function 
necessary  to  perfect  the  adult,  and  therefore,  as  Von  Baer 
put  it,  the  progress  of  development  is  from  the  general  to  the' 
special. 

It  is  upon  a  misconception  of  the  true  import  of  this  law 
that  the  theory  arose,  that  every  animal  in  its  development 
passed  through  a  series  of  stages,  in  which  it  resembles,  in 
turn,  the  different  inferior  members  of  the  animal  scale. 
With  regard  to  man,  standing   at   the   top   of  the  whole 


94  ELEMENTS  OF  BIOLOGY. 

animal  kingdom,  this  theory  has  been  expressed  as  follows : 
— "  Human  organogenesis  is  a  transitory  comparative  ana- 
tomy, as,  in  its  turn,  comparative  anatomy  is  a  fixed  and 
permanent  state  of  the  organogenesis  of  man  "  (Serres).  In 
other  words,  the  embryo  of  a  Vertebrate  animal  was  believed 
to  pass  through  a  series  of  changes  corresponding  respect- 
ively to  the  permanent  types  of  the  lower  sub-kingdoms 
— namely,  the  Protozoa,  Coelenterata,  Annuloida,  Annu- 
losa,  and  Mollusca — before  finally  assuming  the  true  ver- 
tebrate characters.  Such,  however,  is  not  truly  the  case. 
The  ovum  of  every  animal  is  from  the  first  impressed  with 
the  power  of  developing  in  one  direction  only,  and  very 
early  exhibits  the  fundamental  characters  proper  to  its  sub- 
kingdom,  never  presenting  the  structural  peculiarities  be- 
longing to  any  other  morphological  type.  Nevertheless,  the 
differences  which  subsist  between  the  members  of  each  sub- 
kingdom  in  their  adult  condition  are  truly  referable  to  the 
degree  to  which  development  proceeds,  the  place  of  each 
individual  in  his  own  sub-kingdom  being  regulated  by  the 
stage  at  which  development  is  arrested.  Thus,  many  cases 
are  known  in  which  the  younger  stages  of  a  given  animal 
represent  the  permanent  adult  condition  of  an  animal  some- 
what lower  in  the  scale.  Thus,  to  give  a  single  example, 
the  young  of  the  water-breathing  Univalve  Shell-fish  {Gas- 
teropoda) transiently  present  all  the  essential  characters 
which  distinguish  the  adult  condition  of  the  minute  oceanic 
Molluscs  known  as  the  Pteropods.  The  young  Gasteropod, 
namely,  swims  about  freely  by  means  of  two  lobes  or  fins 
attached  to  the  sides  of  the  head  (fig.  24,  A),  and  similar 
•fins  are  present  in  the  Pteropods  in  their  adult  conditicn 
(fig,  24,  B),  enabling  the  animal  to  swim  actively  at  the 
surface  of  the  open  ocean.  The  development  of  the  Gas- 
teropod, however,  proceeds  beyond  the  point,  and  the  adult 
is  much  more  highly  specialised  than  is  the  adult  Pteropod. 
Upon  the  theory  of  "  Evolution  "  such  facts  as  the  above 
would  be  explained  simply  by  the  law  of  hereditary  trans- 


. 


DEVELOPMENT. 


95 


mission.     Upon  this  theory,  the  PtCiOpods  and  the  Gastero- 
pods  have  proceedrd  from  a  common  progenitor,  and  have 


A  B 

Fig.  24.  —  A,  Young  of  Eolis,  a  water-brcalhiiig  Gasteropod,  showing  the  provi- 
sional buccal  lubes.  15,  Adn\l  i'i^ro'pod^Limai iufi  Antarctica).  After  Wood- 
ward. 

therefore  inherited  certain  common  characters.  Since  the 
period,  however,  when  they  branched  off  from  the  common 
stem,  the  Gasieropods  have  undergone  much  modification, 
whereas  the  Pteropods  have  retained  very  much  the  char- 
acters of  the  ( riginal  stock.  The  adult  Gasteropod  comes, 
therefore,  to  differ  very  much  from  the  adult  Pteropod ;  but 
the  jw///^'^  Gasieropod,  being  as  yet  unspecialised,  still  pre- 
sents characters  derived  from  the  primitive  stock  in  an  un- 
modified form. 

Retrograde  Development. — Ordinarily  speaking,  the 
course  of  development  is  an  ascending  one,  and  the  adult  is 
more  highly  organised  than  the  young ;  but  there  are  cases 
in  which  there  is  an  apparent  reversal  of  this  law,  and  the 
adult  is  to  all  appearance  a  degraded  form  as  compared 
with  the  larva.  This  phenomenon  is  known  as  "retro- 
grade "  or  "  recurrent "  development,  and  it  is  seen  in  its 
most  marked  form  in  animals  which  lead  a  free  life  when 
young,  but  are  parasitic  in  their  habits  when  fully  grown, 
though  it  is  not  exclusively  confined  to  these.  A  striking 
example  of  retrograde  development  is  afforded  by  the  singu- 
lar crustaceans  known  as  Epizoa.  In  these  the  larval  form 
is  free-swimming,  provided  with  locomotive  limbs,  and  fur- 
nished with  well-developed  organs  of  vision,  being  in  most 


96 


ELEMENTS  OF   BIOLOGY. 


respects  similar  to  the  permanent  condition  of  certain  other 
Crustaceans  (such  as  the  little  Cypridcs).     The  adult,  how- 


A  B 

Fig.  25. — A,  Young  of  one  of  the  Efiizrf/t  {Acktfterfs).     B,  Swollen  and  deformed 
adult  of  one  of  the  Epizoa  (Leyiueu). 

ever,  is  in  these  cases  more  or  less  swollen  and  deformed, 
degraded  into  a  completely  sedentary  animal,  more  or  less 
comph  Ay  deprived  of  organs  of  sense,  and  leading  an 
almost  \  ^etative  life.  As  a  compensation,  however,  organs 
of  reproci  tion  are  developed  in  the  shapeless  adult,  and 
it  is  in  this  respect  superiqf  to  the  locomotive  but  sexless 
Jarva. 


.^•'■. 


, ,.»,., 


I .  ' 


CHAPTER     X. 


REPRODUCTION 


Reproduction  is  the  process  whereby  new  individuals  are 
generated,  and  the  perpetuation  of  the  sj>ea'es  is  insured  in 
spite  of  the  constant  deaths  of  its  component  members. 
The  modes  in  which  this  end  may  be  attained  exhibit  a 
good  doal  of  diversity,  but  they  may  be  all  considered  under 
two  heads. 

I.  Sexual  Reproduction. — This  consists  essentially  in  the 
production  of  two  distinct  elements,  a  germ-cell  or  ovum, 
and  a  sperm-cell  or  spermatozoid,  by  the  contact  of  which 
the  ovum,  now  said  to  be  "fecundated" — is  enabled  to 
develop  itself  into  a  new  individual.  As  a  rule,  the  germ- 
cell  is  produced  by  one  individual  (female),  and  the  sper- 
matic element  by  another  (male) ;  in  which  case  the  sexes 
are  said  to  be  distinct,  and  the  species  is  said  to  be  "dice- 
cious."  In  other  cases  the  same  individual  has  the  power 
of  producing  both  the  essential  elements  of  reproduction; 
in  which  case  the  sexes  are  said  to  be  united  and  the  indi- 
vidual is  said  to  be  "  hermaphrodite,"  "  androgynous,"  or 
"monoecious."  In  the  case  of  hermaphrodite  animals, 
however,  self-fecundation — contrary  to  what  might  have  been 
expected — rarely  constitutes  the  reproductive  process  j  and, 
as  a  rule,  the  reciprocal  union  of  two  such  individuals  is 
necessary  for  the  production  of  young.      Even  amongst 


98  ELEMENTS  OF  BIOLOGY. 

hermaphrodite  pl.Mits,  where  self-fecundation  may,  and  cer- 
tainly does,  occur,  provisions  .^em  to  exist  by  which  per- 
petual self-fertilisation  is  prevented,  and  the  influence  of 
another  indiviclual  secured  at  intervals.  Amongst  the  higher 
animals  sexual  i  'production  is  the  only  process  whereby 
new  individuals  ciui  be  generated. 

II.  Nonsexual  Reproduction. — Amongst  the  lower  animals 
fresh  beings  may  be  produced  without  the  contact  of  an 
ovum  and  a  spermatozoid ;  that  is  to  say,  without  any  true 
generative  act.  The  processes  by  which  this  is  effected 
vary  in  different  animals,  and  arc  all  spoken  of  as  forms  of 
"  asexual "  or  '*  agamic  "  reproduction.  As  Ave  .shall  see, 
however,  the  true  "  individual "  is  verv  rarely  produced  other 
wise  than  sexually,  and  most  forms  of  agamic  reproduction 
are  really  modifications  of  growth. 

a.  Gemmation  and  Fission. — Gemmation,  or  budding, 
consists  in  the  production  of  a  bud,  or  buds,  generally  from 
the  exterior,  but  sometimes  from  the  interior,  of  the  body 
of  an  animal,  which  buds  are  developed  into  independent 
beings,  which  may  or  may  not  remain  permanently  attached 
to  the  parent  organism.  Fission  differs  from  gemmation 
solely  in  the  fact  that  the  new  structures  in  the  former  case 
are  produced  by  a  division  of  tlie  body  of  the  original 
organism  into  separate  parts,  whicli  may  remain  in  connec- 
tion, or  may  undergo  detachment. 

The  simplest  form  of  gemmation,  perhaps,  is  seen  in  the 
power  possessed  by  certain  animals  of  reproducing  parts  of 
their  bodies  which  they  may  have  lost.  Thus,  the  Crus- 
tacea possess  the  power  of  reproducing  a  lost  limb,  by 
means  of  a  bud  which  is  gradually  developed  till  it  assumes 
the  form  and  takes  the  place  of  the  missing  member.  In 
these  cases,  however,  the  process  is  not  in  any  way  genera- 
tive, and  the  product  of  gemmation  can  in  no  sense* be 
spoken  of  as  a  distinct  being  (or  zooid). 

Another  form  of  gemmation  may  be  exemplified  by  what 
takes  place  in  the  Foraminifera,  one  of  the  classes  of  the 


REPRODUCTION. 


99 


Protozoa  (fig.  26).  The  primitive  form  of  a  Foraminifer 
is  simply  a  little  sphere  of  sarcode,  which  has  the  power  of 
secreting  from  its  outer  surface  a  calcareous  envelope;  ond 
this  condition  may  be  permanently  retained  (as  in  Lagena, 
fig.  26,  A).  In  other  cases  a  process  of  budding  or  gemma- 
tion takes  place,  and  the  primitive  mass  of  sarcode  produces 
from  itself,  on  one  side,  a  second  mass  exactly  similar  to 
the  first,  which  does  not  detach  itself  from  its  parent,  but 
remains  permanently  connected  with  it.  This  second  mass 
repeats  the  process  of  gemmation  as  before,  and  this  goes 
on — all  the  segments  remaining  attached  to  one  another — 
until  a  body  is  produced,  which  consists  of  a  number  of 
little  spheres  of  sarcode  in  organic  connection  with  one 


Fig.  26. — Diagram  to  illustrate  the  formation  of  the  compound  Foraminiffra.  A, 
Simple  form  {Lagena),  consisting  of  a  sphere  of  sarcode,  surrounded  by  a  cal- 
careous shell ;  B,  Compound  form,  produced  by  linear  gemmation  from  a  primi- 
tive segment  resembling  A  (Nodosaria)  ;  C,  Compound  form  {Discorbina),  in 
which  the  buds  arc  thrown  out  in  a  spiral,  the  coils  of  which  lie  in  one  plane. 

another,  and  surrounded  by  a  shell,  often  of  the  most  com- 
plicated description.  In  this  case,  however,  the  buds  pro- 
duced by  the  primitive  spherule  are  not  only  not  detached, 
but  they  can  only  remotely  be  regarded  as  independent 
beings.  They  are,  in  all  respects,  identical  with  the  prim- 
ordial segment,  and  it  is  rather  a  cr  of  "  vegetative  ''  re- 
petition of  similar  parts. 

Another  form   of  gemmation  is  exhibited   in   such   an 


m 


lOO 


ELEMENTS  OF   BIOLOGY. 


organism  as  tho  common  sea-mat  (Flustra),  which  is  a  com- 
posite orgai>ism  composed  of  a  multitude  of  similar  beings, 
each  of  which  inha)its  a  little  chamber  or  cell;  the  whole 
forming  a  structure  not  unlike  a  sea-weed  in  appearance 
(fig.  27).     This  colony  is  produced  by  gemmation  from  a 


Fig.  97. — Flustra  hispida,  one  of  the  Sea-mats,  a  Portion  of  the  colony,  natura 
size  ;  b  A  fragment  magnified,  to  show  the  cells  in  which  the  separate  polypidcs 
are  contained. 

single  primitive  being  ("  polypide  "),  which  throws  out  buds, 
each  of  which  repeats  the  process,  apparently  almost  inde- 
finitely. All  the  buds  remain  in  contact  and  connected 
with  one  another,  but  each  is,  nevertheless,  a  distinct  and 
independent  being,  capable  of  performing  all  the  functions 
of  life.  In  this  case,  therefore,  each  one  of  the  innumer- 
able buds  becomes  an  independent  being,  similar  to,  though 


REPRODUCTION. 


lOI 


not  detached  from,  the  organism  which  gave  it  birth.  'J'his 
fs  an  instance  of  wiut  is  called  "  continuous  gemmation." 

In  other  cases — as  in  the  common  fresh-water  polype  or 
Hydra  (fig.  8)  —  the  budi  uliich  are  thrown  out  by  the 
primitive  organism  become  developed  into  creatures  exactly 
resembling  the  parent;  but,  inslrad  of  remaining  permanently 
attached,  and  thus  giving  rise  to  a  compound  organism,  they 
are  detached,  to  lead  an  entirely  independent  existence. 
This  is  a  simple  instance  of  what  is  termed  "  discontinuous 
gemmation." 

The  method  and  results  of  fission  may  be  regarded  as 
essentially  the  same  as  in  the  case  of  gemmation.  The 
products  of  the  division  of  the  body  of  the  primitive  organ- 
ism may  either  remain  undetached,  when  they  will  give  rise 
to  a  composite  structure  (as  in  many  corals),  or  they  may 
be  thrown  off  and  lead  an  independent  existence  (as  in  some 
of  the  Hydrozoa). 

An  excellent  example  of  simple  discontinuous  fission  is 


«— 5 


-J... 


Fig.  28. — A,  Paramcccium,  showing  the  nucleus  («)  and  two  contractile  vesicles  (p)- 
B,  Farc.viccciiini  bursaria  {tSx&x  Stein),  dividing  transversely:  n  Nucleus;  «' 
Nucleolus  ;  v  Contractile  vesicle.     C,  Paramaeciutn  aurelia  (after  Ehrenberg), 
dividing  longitudinally. 

»  ■  ■ '   ,  ■ 

afforded  by  the  common  animalcule,  Paramcecium  (fig.  28). 
Iliis  little  creature  produces  fresh  beings  by  a  process  of 


102 


KI.KMKNTS  OF  DIOLOCiY. 


self-division  or  cleavage,  which  may  take  place  either  trans- 
versely or  longitudinally.  In  either  case  a  groove  is  formed 
on  the  exterior  surface,  which  gradually  deepens,  till  the 
original  organism  is  split  up  into  two  similar  and  indepen- 
dent Parama'cia.  It  would  appear,  however,  that  the  initia- 
tive in  the  process  of  fission  is  taken  by  the  reproductive 
organs  in  the  interior  of  the  body,  which  first  divide  into 
two  similar  halves. 

We  are  now  in  a  position  to  understand  what  is  meant, 
strictly  speaking,  by  the  term  **  individual."  In  zoological 
language,  an  individual  is  defined  as  "  equal  to  the  total 
result  of  the  development  of  a  single  ovupn."  Amongst  the 
higher  animals  there  is  no  difficulty  about  this,  for  each 
ovum  gives  rise  to  no  more  than  one  single  being,  which  is 
incapable  of  repeating  itself  in  any  other  way  than  by  the 
production  of  another  ovum ;  so  that  an  individual  is  a 
single  animal.  It  is  most  important,  however,  to  compre- 
hend that  this  is  not  necessarily  or  always  the  case.  In 
such  an  organism  as  the  sea-mat  (fig.  27),  the  ovum  gives 
rise  to  a  primitive  polypide  which  repeats  itself  by  a  process 
of  continuous  gemmation,  until  an  entire  colony  is  produced, 
each  member  of  which  is  independent  of  its  fellows,  and  is 
capable  of  producing  ova.  In  such  a  case,  therefore,  the 
term  "individual"  must  be  applied  to  the  entire  colony, 
since  this  is  the  result  of  the  development  of  a  single  ovum. 
The  separate  beings  which  compose  the  colony  are  techni- 
cally called  "zooids."  In  like  manner,  the  Hydra  which 
produces  fresh  and  independent  Ilydrje  by  discontinuous 
gemmation,  is  not  an  "  individual,"  but  is  a  zooid.  Here 
the  zooids  are  not  permanently  united  to  one  another,  and 
the  "  individual "  Hydra  consists  really  of  the  primitive 
Hydra,  plus  all  the  detached  Hydrae  to  which  it  gave  rise. 
In  this  case,  therefore,  the  "  individual "  is  composed  of  a 
number  of  disconnected  and  wholly  independent  beings,  all 
of  which  are  the  result  of  the  development  of  a  single  ovum. 
It  is  to  be  remembered  that  both  the  parent  zooid  and  the 


RKPRODUCTION. 


103 


*'  produced  zooids "  are  capable  of  giving  rise  to  fresh 
Hydrae  by  a  true  generative  process.  It  must  also  be  borne 
in  mind  that  this  production  of  fresh  zooids  by  a  process  of 
gemmation  is  not  so  essentially  different  to  the  tnie  sexual 
process  of  reproduction  as  might  at  first  sight  appear,  since 
the  ovum  itself  may  be  regarded  merely  as  a  highly-spe- 
cialised bud.  In  the  Hydra,  in  fact,  where  the  ovum  is 
produced  as  an  external  process  of  the  wall  of  the  body; 
this  likeness  is  extremely  striking.  The  ovarian  Inid,  how- 
ever, differs  from  the  true  gemmaj  or  buds  in  its  inability  to 
develop  itself  into  an  independent  organism,  unless  pre- 
viously brought  into  contact  with  another  special  generative 
element.  The  only  exceptions  to  this  statement  are  in  the 
rare  cases  of  true  "parthenogenesis,"  to  be  ^bsequently 
alluded  to. 

b.  Reproduction  by  Internal  Gemmation. — Before  consider- 
ing the  phenomena  of  *'  alternate  generations,"  it  will  be  as 
well  to  glance  for  a  moment  at  a  peculiar  form  of  gemma- 
tion exhibited  by  some  of  the  Polyzoa,  which  is  in  some 
respects  intermediate  between  ordinary  discontinuous  gem- 
mation and  alternation  of  generations.  These  organisms 
are  nearly  allied  to  the  sea-mat,  already  spoken  of,  and, 
like  it,  can  reproduce  themselves  by  continuous  gemmation 
(forming  colonies),  by  a  true  sexual  process,  and  rarely  by 
fission.  In  addition  to  all  these  methods  they  can  repro- 
duce themselves  by  the  fCiVnation  of  peculiar  internal  buds, 
which  are  called  "  statoblasts."  These  buds  are  developed 
upon  a  peculiar  cord,  which  crosses  the  body-cavi.y,  and  is 
attached  at  one  end  to  the  fundus  of  the  stomach.  When 
mature  they  drop  off  from  this  cord,  and  lie  loose  in  the 
cavity  of  the  body,  whence  they  are  liberated  on  the 
death  of  the  parent  organism.  When  thus  liberated, 
the  statoblast,  after  a  longer  or  shorter  i)eriod,  ruptures 
and  gives  exit  to  a  young  Polyzoon,  which  has  essentially 
the  same  structure  as  the  adult.  It  is,  however,  simple, 
and  has  to  undergo  a  process  of  continuous  gemmation 


104 


ELEMENTS  ^^  BIOLOGY. 


before  it  can  assume  the  compound  form  proper  to  the 
adult. 

As  regards  the  nature  of  these  singular  bodies,  "  the  in- 
variable absence  of  gernnnal  vesicle  and  germinal  spot,  and 
their  never  exhibiting  the  phenomena  of  yelk-cleavage,  in- 
dependently of  the  conclusive  fact  that  true  ova  and  ovary 
occur  elsewhere  in  the  same  indi\  idual,  are  cjuite  decisive 
against  their  being  eggs.  We  must  then  look  upon  them 
as  gemma  peculiarly  encysted,  and  destined  to  remain  for  a 
period  in  a  quiescent  or  pupa-like  state." — (Allman). 

c.  Alternation  of  Generations. — In  the  case  of  the  Hydra 
and  the  sea-mat,  which  we  have  considered  above,  fresh 
zooids  are  produced  by  a  primordial  organism  by  gemma- 
tion ;  the  beings  thus  produced  (as  well  as  the  parent)  being 
capable  not  only  of  repeating  the  gemmiparous  process,  but 
also  of  producing  new  individuals  by  a  true  generative  act. 
We  have  now  to  consider  a  much  more  complex  series  of 
phenomena,  in  which  the  organism  which  is  developed  from 
the  primitive  ovum  produces  by  gemmation  two  sets  of 
zooids,  one  of  which  is  destitute  of  sexual  organs,  and  is 
capable  of  performing  no  other  function  than  that  of  nutri- 
tion, whilst  the  other  is  provided  with  reproductive  organs, 
and  is  destined  for  the  perpetuation  of  the  species.  In  the 
former  case  the  produced  zooids  all  resembled  each  other, 
and  the  parent  organism  which  gave  rise  to  them ;  in  the 
latter  case,  the  produced  zooids  are  often  utterly  unlike 
each  other  and  unlike  the  parent,  lince  their  functions  are 
entirely  different. 

The  simplest  form  of  the  process  is  seen  in  certain  of  the 
Hydroid  Zoophytes,  such  as  Hydractinia  (fit?;.  29).  The 
embryo  of  Hydractinia  emerges  from  the  egg  as  a  free- 
swimming  ciliated  body,  which,  after  a  short  locomotive 
existence,  attaches  itself  to  some  marine  object,  develops  a 
mouth  and  tentacles,  and  commences  to  produce  a  colony 
of  zooids  like  itself,  by  a  process  of  continuous  gemmation. 
The  zooids  thus  produced  remain  permanently  in  connec- 


REPRODUCTION. 


105 


tion  with  one  another,  with  the  result  that  a  compound 
organism  is  produced,  consisting  of  a  collection  of  nutritive 
factors  or  *'  polypites,"  organically  united,  but  enjoying  a 
semi-independent  existence.     In  this  phase  of  its  life  we 


Fig.  29. — Group  of  zo^iids  nK  Hydractinia  echtnata.     Enlarged  (after  Hincks).    a  a 
Nutritive  zodids;  bb  Generative  zoOids,  carrying  sacs  filled  with  ova. 


may  compare  Hydractiuia  with  a  tree  composed  of  numer- 
ous leaf-buds  borne  upon  a  branched  stem,  but  not  yet 
exi:ibiting  flowers.  Such  a  comparison  would  involve  some- 
thing more  than  a  mere  superficial  resemblance.  The 
ordinary  zooids  of  Hydractinia  are  produced  by  a  process 


I06  ELJ£MENTS  OF   BIOLOGY. 

of  budding,  remain  connected  to  one  another,  and  have 
no  power  of  producing  the  essential  elements  of  reproduc- 
tion. Further,  each  zooid  has  to  contribute  to  the  nourish- 
ment of  the  colony  as  a  whole,  at  the  same  time  that  its  liie 
is,  to  a  limited  extent,  independent  of  that  of  the  other 
members  of  the  growth.  Lastly,  the  life  of  the  colony  is  in 
no  way  dependent  upon  the  life  of  its  individual  factors,  but 
the  polypites  may  be  destroyed  or  may  die,  and  the  general 
stem  may  yet  retain  its  vitality,  and  may  recommence  the 
process  of  budding.  Similarly,  the  leaves  of  a  tree  are  pro- 
duced by  a  process  of  continuous  gemmation,  remain  per- 
manently connected,  and  have  no  power  of  sexual  repro- 
duction. They  are  nutritive  factors  of  a  common  growth, 
to  the  m.iintenance  and  development  of  which  they  minis- 
ter ;  and  the  existence  of  the  tree  is  in  no  way  limited  by 
the  life  of  any  individual  leaf. 

This  comparison,  however,  may  be  carried  still  further 
without  breaking  down.  The  ordinary  leaf-buds  of  the 
tree  are  in  no  way  connected  with  reproduction ;  and 
whilst  the  tree  may  increase  considerably,  as  an  individual, 
by  the  constant  formation  of  fresh  buds,  it  has  no  power 
of  perpetuating  its  species  so  long  as  it  merely  produces 
leaves.  At  certain  periods,  however,  the  tree  produces 
special  buds  or  flowers,  in  which  are  developed  the  essen- 
tial elements  of  reproduction,  by  the  union  of  which  a  seed 
is  produced,  from  which,  under  suitable  conditions,  a  young 
tree  will  spring.  Not  only  is  this  the  case,  but  we  have  the 
remarkable  fact  that  che  flowers  or  reproductive  buds  of  the 
tree  are  morphologically  identical  with  the  leaf-buds  or  nutri- 
tive buds ;  whilst  the  difference  of  function  causes  such  a 
difference  of  structure  that  the  morphological  unity  of  the 
two  can  only  with  some  difficulty  be  recognised.  Similarly, 
in  Hydractinia,  the  ordinary  zooids  of  the  colony  have  na 
reproductive  organs ;  and  though  there  is  theoretically  no 
limit  to  the  size  which  the  organism  may  reach  by  gemma- 
tion, its  buds  are  not  detached,  and  the  species  would  die 


« 


REPRODUCTION. 


107 


out,  unless  some  special  provision  were  made  for  its  preser- 
vation. Besides  the  nutritive  zooids,  however,  other  buds 
are  produced,  which  are  morphologically  identical  with  the 
former,  uJt  which  are  greatly  modified  for  the  purpose  of 
producing  the  essential  elements  of  reproduction  (fig.  29,  b). 
These  "generative  zooids"  derive  their  nourishment  from 
the  materials  collected  by  the  nutritive  zooids,  since  they 
are  incapable  of  obtaining  food  for  themselves.  Ultimately, 
the  elements  of  reproduction  are  developed,  and  the  fer- 
tilised ova  give  rise  to  ciliated  embryos,  similar  to  the  one 
with  which  the  cycle  began. 

In  this  case,  ther«'?fore,  the  "  individual "  Hydradinia 
consists  of  a  series  of  nutritive  zooids,  collectively  called 
the  "  trophosome,"  and  another  series  of  reproductive 
zooids,  collectively  called  the  "  gonosome,"  the  two  groups 
differing  from  one  another  in  form,  but  remaining  in  organic 
connection. 

In  other  Hydroid  Zoophytes  nearly  allied  to  Hydractinia^ 
the  process  advances  a  step  further,  and  we  arrive  at  pheno- 
mena which  we  cannot  parallel  with  anything  we  observe  in 
plants.  Up  to  a  certain  point,  however,  the  phenomena 
agree  with  those  just  described  in  Hydradinia.  Thus,  in 
Clytia  {Campa?iularia)  we  have  a  rooted  colony  or  "  tropho- 
some "  composed  of  a  number  of  nutritive  zooids  produced 
by  continuous  gemmation,  and  remaining  organically  con- 
nected (^'^.  30).  The  members  of  this  colony  have  no 
power  ot  maturng  the  elements  of  reproduction ;  but  the 
organism  at  certain  seasons  produces  large  oval  horny 
sacs  (fig.  30,  g),  in  which  generative  zooids  are  developed. 
These  gererative  zooids,  however,  do  not  produce  the 
generativ.i  elements  so  long  as  they  remain  attached  to 
the  pan  nt  colony ;  but  they  require  a  preliminary  period 
of  independent  existence.  For  this  purpose  they  are  spe- 
cially organised,  and  when  sufficiently  matured  they  are 
liberated  from  their  containing  capsules,  and  are  detached 
from   the    stationary  colony.       The    liberated   generative 


io8 


ELEMENTS  OF  BIOLOGY. 


zooids  now  appear  as  entirely  independent  beings,  which 
are  known  as  Jelly-fishes,  and  which  are  so  unlike  the 
colony  from  which  they  spring  that  they  were  originally 
described  as  distinct  animals.      Each  generative  zooid  or 


Fig.  JO. — Portion  of  the  colony  oiClytia  {Cavt^anularia)  yohnstont,  mag^nified ;  / 
I^ utritive zouid ;  ^Capsules  in  which  the  reproductive  zofiids  are  produced. 

"  mediisoid"  (fig.  31)  consists  of  a  little  transparent  gl  iy 
disc  or  bell,  from  the  under  surface  of  which  there  is  sus- 
pended a  modified  zooid  or  "  polypite,"  in  the  form  of  a 


REPRODUCTION. 


109 


central  process,  which  is  known  by  the  name  of  the  "  manu- 
brium." 

The   whole   organism    swims 
gaily    through    the    water,    pro- 
pelled by  the  contractions  of  the 
bell  or  disc  {gonocalyx) ;  and  no 
one  would  now  suspect  that  it 
was  in  any  way  related   to  the 
fixed  plant -like  zoophyte   from 
which  it  was  originally  budded 
off.     The  central  polypite  is  fur- 
nished with  a  mouth  at  its  distal 
end,  and  the  mouth  opens  into  a 
digestive  sac.     From  the  proxi- 
mal end  of  this  stomach  proceed 
four  radiating  canals  which  ex- 
tend to  the  circumference  of  the 
disc,  where  they  all  open  into  a 
single  circular  vessel  surrounding 
the  mouth  of  the  bell.     From  the 
margins  of  the   disc   hang  also 
a  number  of  delicate   extensile 
filaments  or  tentacles ;   and  the 
circumference     is     still    further 
adorned  with  a  series  of  brightly- 
coloured    spots,  which  are  pro- 
bably organs  of  sense.      The  mouth  of  the  bell  is  par- 
tially closed  by  a  delicate  transparent  membrane  or  shelf, 
the   so-called   "veil."      Thus   constituted,  these  beautiful 
little  beings  lead  an  independent  and  locomotive  ex'.stence 
for  a  longer  or  shorter  period.      Ultimately,  'he  essential 
elements  of  reproduction  are  developed  in  special  organs, 
situated  in  the  course  of  the  radiating  canals  of  the  disc. 
The  resulting  embryos  are  ciliated  and  free-swimming,  but 
ultimately  fix  themselves,  and  develop  into  the  plant-like 
colony  from  which  fresh  medusoids  may  be  budded  off. 
6 


Fig.  31.  —  Free  mcdusiform  gono- 
phorc  of  Clytia  Johnstoni  (after 
Hincks).  a  Central  polypite  or 
manubrium  ;  b  k  Radiatmg  gas- 
tro- vascular  carftils  ;  c  Circular 
canal ;  tn  Marginal  bodies ;  t 
Tentacles. 


no 


ELEMENTS  OF  BIOLOGY. 


For  these  phenomena  we  can  find  no  parallel  amongst 
plants.  If  we  imagine,  however,  a  tree  which  could  detach 
its  flowers,  and  if  we  suppose  these  to  be  organised  for  an 
independent  existence,  and  to  be  capable  of  increasing  in 
size  after  their  liberation,  we  should  have  very  much  the 
state  of  things  which  we  observe  in  Clytia. 

Still  more  extraordinary  phenomena  have  been  observed 
in  some  others  of  the  Hydrozoa,  as  in  the  Lucernarida.  In 
these,  the  egg  gives  rise  to  a  minute,  free-swimming,  ciliated 
body  (fig.  32,  a),  which  consists  of  two  layers  enclosing  a 
central  cavity.  Soon  it  becomes  pear-shaped,  fixes  itself  to 
some  solid  body  by  its  tarering  extremity,  and  develops  a 
mouth  and  tentacles  at  the  other  extremity.  It  is  now 
known  as  the  Hydra-tuba  (fig.  32),  from  its  resemblance  in 
form  to  the  fresh-water  polype  or  Hydra.  The  Hydra-tuba 
has  the  jwwer  of  multiplying  itself  by  gemmation,  and  it 


Fig.  32.— Development  of  one  of  the  Lucernarida  {A  urelia).  a  Free-swimming  cili- 
ated embryo ;  A  Hydra-tuba;  c  Hydra-tuba  undergoing  transverse  fission; 
d  The  same  with  the  fission  further  advanced. 

can  produce  extensive  colonies  in  this  way ;  but  it  does  not 
obtain  the  power  of  generating  the  essential  elements  of  re- 
production. Under  certain  circumstances,  however,  the 
Hydra-tuba  enlarges,  and  its  body  becomes  constricted  by  a 


REPRODUCTION. 


Ill 


series  of  transverse  annulations  or  grooves  (fig.  32,  c).  These 
grooves  go  on  deepening,  and  the  segments  which  they  mark 
otf  become  deeply  lobed  and  incised  at  their  margins,  till 
the  whole  organism  assumes  the  aspect  of  a  pile  of  saucers 
arranged  one  upon  another  with  their  concave  surfaces  up- 
wards. A  new  set  of  tentacles  is  developed  near  the  base  of 
th"  organism,  and  all  the  segments  above  this  point  gradu- 
ally fall  off,  and  swim  away  to  lead  a  free  life.  These  libe- 
rated segments  of  the  little  Hydra-tuba  (it  is  about  half  an 
inch  in  height)  now  lead  an  independent  existence,  and 
were  originally  described  by  naturalists  as  distinct  animals 
(fig.  33).     They  are  provided  with  a  swimming-bell  or  "  urn- 


Fig.  33. — Hidden-eyed  Medusae.     Generative  20oid  of  one  of  the  Lucemarida 
(Chrysaora  hysoscella).    After  Gosse. 

brella,"  by  the  contractions  of  which  they  are  propelled 
through  the  water.  From  the  centi^  of  the  umbrella  is  sus- 
pended a  modified  polypite  with  lobed  and  scalloped  lips ; 


112  ELEMENTS  OF  BIOLOGY. 

and  the  margins  of  the  bell  carry  organs  of  rense  and  long 
tentacles.  The  central  polypite  hay  a  mouth  and  digestive 
cavity,  leading  into  a  complex  canal-system.  At  first  of 
small  size,  they  feed  eagerly,  and  increase  largely  in  bulk, 
in  some  cases  attaining  perfectly  colossal  dimensions  (as 
much  in  one  species  as  twenty  Teet  in  circumference).  After 
a  while  they  develop  the  essential  elements  of  reproduction, 
and  after  the  fecundation  and  liberation  of  their  ova,  they 
die.  The  ova,  however,  are  not  developed  into  the  free- 
swimming  and  comparatively  gigantic  organism  by  which 
they  were  immediately  produced,  but  into  the  minute,  fixed, 
sexless  Hydra-tuba. 

We  thus  see  that  a  small,  sexless  zooid,  which  is  capable 
of  multiplying  itself  by  gemmation,  produces  by  fission 
several  independent  locomotive  beings,  which  are  capable 
of  nourishing  themselves  and  of  performing  all  the  functions 
of  life.  In  these  are  produced  generative  elements,  which 
give  rise  by  their  development  to  the  little  fixed  creature 
with  which  the  series  began. 

To  the  group  of  phenomena  of  which  the  above  are  ex- 
amples, the  name  "alternation  of  generations"  was  applied 
by  Steenstrup ;  but  the  name  is  not  an  appropriate  one, 
since  the  process  is  truly  an  alternation  of  generation  with 
gemmation  or  fission.  The  oi]ly  generative  act  takes  place 
in  the  reproductive  zooid,  and  the  production  of  this  from 
the  nutritive  zooid  is  a  process  of  gemmation  or  fission,  and 
not  a  process  of  generation.  The  "  individual,"  in  fact,  in 
all  thc:se  cases,  must  be  looked  upon  as  a  double  being 
composed  of  two  factors,  both  of  which  lead  more  or  less 
completely  independent  lives,  the  one  being  devoted  to 
nutrition,  the  other  to  reproduction.  The  generative  being, 
however,  is  in  many  cases  not  at  first  able  to  mature  the 
sexual  elements,  and  is  therefore  provided  with  the  means 
necessary  for  its  growth  and  nourishment  as  an  independent 
organism.  It  must  alsa  be  remembered  that  the  nutritive 
half  of  the  "individual"  is  usually,  and  the  generative  half 


REPRODUCTION. 


113 


somelimes,  compoutid ;  that  is  to  say,  composed  of  a  number 
of  zooids  produced  by  gemmation ;  so  that  the  zoological 
individual  in  these  cases  becomes  an  extremely  complex 
being. 

These  phenomena  of  so-called  "alternation  of  generations," 
or  "  metagenesis,"  occur  in  their  most  striking  form  amongst 
the  Hydrozoa ;  but  they  occur  also  amongst  some  of  the 
intestinal  worms  (Entozoa),  and  amongst  some  of  the 
Tunicata  (Molluscoida). 

d.  Parthenogenesis. — "  Parthenogenesis  "  is  the  term  cm- 
ployed  to  designate  certain  singular  phenomena,  resulting 
in  the  production  of  new  individuals  by  virgin  females 
without  the  intervention  of  a  male.  By  Professor  Owen, 
who  first  employed  the  term,  parthenogenesis  is  applied 
also  to  the  processes  of  gemmation  and  fission,  as  exhibited 
in  sexless  beings  or  in  virgin  females ;  but  it  seems  best  to 
consider  these  phenomena  separately.  Strictly,  the  tern, 
parthenogenesis  ought  to  be  confined  to  the  production  of 
new  individuals  from  virgin  females  by  means  oi  ova^  which 
are  enabled  to  develop  themselves  without  the  contact  of 
the  male  element.  The  difficulty  in  this  definition  is  found 
in  framing  an  exact  definition  of  an  ovum,  such  as  will  dis- 
tinguish it  from  an  internal  gemma  or  bud.  No  body, 
however,  should  be  called  an  "ovurn"  which  does  not 
exhibit  a  germinal  vesicle  and  germinal  spot,  and  which 
does  not  exhibit  the  phenomenon  known  as  segmentation 
of  the  yelk.  Moreover,  ova  are  almost  invariably  produced 
by  a  special  organ,  or  ovary. 

As  examples  of  parthenogenesis  we  may  take  what  occurs 
in  plant-lice  (Aphides)  and  in  the  honey-bee ;  but  it  will  be 
seen  that  in  neither  of  these  cases  are  the  phenomena  so 
unequivocal,  or  so  well  ascertained,  as  to  justify  a  positive 
assertion  that  they  are  truly  referable  to  parthenogenesis  in 
the  above  restricted  sense  of  the  term. 

The  Aphides,  or  plant-lice  (fig.  34),  which  are  so  com- 
monly found  parasitic  upon  plants,  are  seen  towards  the 


114 


ELEMENTS  OF   BIOLOGY. 


close  of  autumn  to  consist  of  male  and  female  individuals. 
By  ilic  sexual  union  of  these,  true  ova  are  produced,  which 


f'8-  34- — Bean  Aphis  {Aphis  fabcr),  winged  male  and  wingless  female. 

remain  dormant  through  the  winter.  At  the  approach  of 
spring  these  ova  are  hatched;  but  instead  of  giving  birth  to 
a  number  of  males  and  females,  all  the  young  are  of  one 
kind,  variously  regarded  as  neuters,  virgin  females,  or 
hermaphrodites.  Whatever  their  true  nature  may  be,  these 
individuals  produce  viviparously  a  brood  of  young  which 
resemble  themselves ;  and  this  second  generation,  in  like 
manner,  produces  a  third, — and  so  the  process  may  be  re- 
peated, for  as  many  as  ten  or  more  generations,  throughout 
the  summer.  When  the  autumn  comes  on,  however,  the 
viviparous  Aphides  produce — in  exactly  the  same  manner — 
a  final  brood ;  but  .this,  instead  of  being  composed  entirely 
of  similar  individuals,  is  made  up  of  males  and  females. 
Sexual  union  now  takes  place,  and  ova  are  produced  and 
fecundated  in  the  ordinary  manner. 

The  bodies  from  which  the  young  of  the  viviparous 
Aphides  are  produced  are  variously  regarded  as  internal 
buds,  as  "pseudova"  {i.e.,  as  bodies  intermediate  between 
buds  and  ova),  and  as  tme  ova. 

Without  entering  into  details,  it  is  obvious  that  there  is 
only  one  explanation  of  these  phenomena  which  will  justify 
us  in  regarding  the  case  of  the  viviparous  Aphides  as  one 
of  true  parthenogenesis,  as  above  defined.  If,  namely,  the 
spring  broods  are  true  females,  and  the  bodies  which  they 


REPRODUCTION. 


"5 


produce  in  their  interior  are  true  ova,  then  the  case  is  one 
of  genuine  parthenogenesis,  for  there  are  certainly  no  males. 
The  case  might  still  be  called  one  of  parthenogenesis,  even 
though  the  bodies  from  which  these  broods  are  produced 
be  regarded  as  internal  buds,  or  as  *'  pseudova ; "  for  a  true 
ovum  is  essentially  a  bud.  If,  however,  Balbiani  be  right, 
and  the  viviparous  Aphides  are  really  hermaphrodite,  then, 
of  course,  the  phenomena  are  of  a  much  less  abnormal 
character. 

In  the  second  case  of  alleged  parthenogenesis  which  we 
are  about  to  examine — namely,  in  the  honey-bee — the 
phenomena  which  have  been  described  cannot  be  said  to 
be  wholly  free  from  doubt.  A  hive  of  bees  consists  of 
three  classes  of  individuals :  i.  A  "  queen,"  or  fertile 
female;  2.  The  "workers,"  which  form  the  bulk  of  the 
community,  and  are  really  undeveloped  or  sterile  females ; 
and  3.  The  "  drones,"  or  males,  which  are  only  produced 
at  certain  times  of  the  year.  We  have  here  three  distinct 
sets  of  beings,  all  of  which  proceed  from  a  single  fertile 
individual ;  and  the  question  arises,  In  what  manner  are  the 
differences  between  these  produced  ?  At  a  certain  period 
of  the  year  the  queen  leaves  the  hive,  accompanied  by  the 
drones  (or  males),  and  takes  what  is  known  as  her  "  nuptial 
flight "  through  the  air.  In  this  flight  she  is  impregnated 
by  the  males,  and  it  is  immaterial  whether  this  act  occurs 
once  in  the  life  of  the  queen,  or  several  times,  as  asserted 
by  some.  Be  this  as  it  may,  the  queen,  in  virtue  of  this 
single  impregnation,  is  enabled  to  produce  fresh  individuals 
for  a  lengthened  period,  the  semen  of  the  males  being  stored 
up  in  a  receptacle  which  communicates  by  a  tube  with  the 
oviduct,  from  which  it  can  be  shut  off  at  will.  The  ova 
which  are  to  produce  workers  (undeveloped  females)  and 
queens  (fertile  females)  are  fertilised  on  their  passage 
through  the  oviduct,  the  semen  being  allowed  to  escape  into 
the  oviduct  for  this  purpose.  The  subsequent  development 
of  these  fecundated  ova  into  workers  or  queens  depends 


11^  ELEMENTS  OF   IIIOI.OGY. 

entirely  upon  the  form  of  the  cell  into  which  the  ovum  is 
placed,  and  upon  the  nature  of  thp  food  which  is  supplied 
to  the  larva.  So  far  there  is  no  doubt  as  to  the  nature  of 
the  phenomena  which  are  observed.  It  is  asserted,  how- 
ever, by  Dzicrzon  and  Siebold,  that  the  males  or  drones  are 
produced  by  the  (jueen  from  ova  which  she  does  not  allow 
to  come  into  contact  with  the  semen  as  they  pass  through 
the  oviduct.  This  assertion  is  supported  by  the  fact  that  if 
the  communication  between  the  receptacle  for  the  semen 
and  "Ihe  oviduct  be  cut  off,  the  queen  will  produce  nqjjiing 
but  males.  Also,  in  crosses  between  the  common  honey-bee 
and  the  Ligurian  bee,  the  queens  and  workers  alone  exhibit 
any  intermediate  characters  between  the  two  forms,  the 
drones  presenting  the  unmixed  characters  of  the  queen  by 
whom  they  were  produced. 

If  these  observations  are  to  be  accepted  as  established — 
and,  upon  the  whole,  there  can  be  little  hesitation  in  accepting 
them  as  in  the  main  correct — then  the  drones  are  produced 
by  a  tnie  process  of  parthenogenesis ;  but  some  observers 
maintain  that  the  development  of  any  given  ovum  into  a 
drone  is  really  due — as  in  the  case  of  the  queens  and 
workers  —  to  the  special  circumstances  under  which  the 
larva  is  brought  up.* 

There  are  various  other  cases  in  which  parthenogenesis 
is  said  to  occur  '  it  the  above  will  suffice  to  indicate  the 
general  charac  cc  of  the  phenomena  in  question.  The 
theories  of  parthenogenesis  appear  to  be  too  complex  to  be 
introduced  here ;  and  there  is  the  less  to  regret  in  their 
omission,  as  naturalists  have  not  yet  definitely  adopted  any 

*  In  the  case  of  Polisks  Gallica,  VoH  Siebold  appears  to  have  proved 
beyond  reasonable  doubt  that  the  males  are  produced  by  a  process  of 
parthenogenesis.  Landois,  however,  asserts  that  the  eggs  of  insects  are 
of  no  sex  ;  that  sex  is  only  developed  in  the  larva  after  its  emergence 
from  the  egg  ;  and  that  in  each  individual  larva  the  sex  is  determined 
■wholly  by  the  nature  of  the  food  u  oon  which  it  is  brought  up — abundant 
nourishment  producing  females,  aiid  scanty  diet  giving  rise  to  males. 


REPRODUCTION. 


117 


one  explanation  of  the  phenomena  to  the  exclusion  of  the 
rest.  • 

e.  Latu  of  Qiiatrc/a^cs. — From  the  phenomena  of  asexual 
reproduction  in  all  its  forms,  M.  de  Quatrcfages  has  de- 
duced the  following  generalisation  : — 

"  The  formation  of  new  individuals  may  take  place,  in 
some*  instances,  by  gemmation  from,  or  division  of,  the 
parent  being;  but  this  process  is  an  exhaustive  one,  and 
cannot  be  carried  out  indefinitely:  when,  therefore,  it  is 
necessary  to  insure  the  continuance  of  the  species,  the  sexes 
must  present  themselves,  and  the  germ  and  sperm  must  be 
allowed  to  come  in  contact  with  one  another." 

It  should  be  added  that  the  act  of  sexual  reproduction, 
though  it  insures  the  perpetuation  of  <he  species^  is  very 
destructive  to  the  life  of  the  individual.  The  formation  of 
the  essential  elements  of  reproduction  appears  to  be  one  of 
the  highest  physiological  acts  of  which  the  organism  is  cap- 
able, and  it  is  attended  with  a  corresponding  strain  upon 
the  vital  energies.  In  no  case  is  this  more  strikingly  exhib- 
ited than  in  the  majority  of  insects,  which  pass  the  greater 
portion  of  their  existence  in  a  sexually  immature  condition, 
and  die  almost  immediately  after  they  have  become  sexually 
perfect,  and  have  consummated  the  art  whereby  the  per- 
petuation of  the  species  is  secured. 

Thus,  as  pointed  out  by  Dr  Carpenter,  and  strongly  in- 
sisted upon  by  Mr  Herbert  Spencer,  we  are  to  regard  sex- 
ual reproduction  as  being  directly  antagonistic  to  nutrition. 
This  brings  us  to  the  further  law  that  the  life  of  an  animal 
whilst  sexually  immature  is  generally  associated  with  active 
growth ;  but  that  when  once  the  generative  expenditure  has 
commenced,  the  nutritive  powers  can  rarely  do  more  than 
maintain  the  organism  in  statu  qiio^  whilst  they  njay  even 
fall  short  of  this.  If  we  regard  the  asexual  methods  of  re- 
production as  being  merely  forms  of  growth,  we  can  readily 
understand  how  it  is  that  zooidal  multiplication  generally 
excludes  sexual  reproduction  for  a  time.     The  time,  how- 


ii8 


ELEMENTS  OF   BIOLOGY. 


ever,  ultimately  comes  in  the  life  of  all  organisms  when  mul- 
tiplication by  gemmation  and  fission  becomes  insufficient, 
when  it  becomes  necessary  that  the  essential  elements  of 
reproduction  should  be  proc'.jced.  The  additional  tax  thus 
imposed  upon  the  organism  is  usually  borne  without  injury  for 
a  certain  length  ot  tim*?  ;  but  the  losses  thus  caused,  if  slow, 
are  sure,  and  in  some  cases  they  are  so  great  as  to  'end  in 
the  immediate  extinction  of  the  organism.  There  are,  how- 
ever, strong  grounds  for  the  belief  that  in  this  respect  man's 
position  differs  materially  from  that  of  all  other  animals. 


CHAPTER    XI. 


REPRODUCTION     IN     PLANTS. 


Having  treated  at  some  length  of  the  reproductive  process 
in  animals,  there  remains  little  that  need  be  said  as  to  the 
reproduction  of  plants.  As  amongst  animals,  plants  exhibit 
both  sexual  and  non-sexual  methods  of  reproduction,  though 
the  peculiarities  of  vegetables  render  the  latter  much  less  con- 
spicuous than  in  animals,  and,  indeed,  usually  lead  to  their 
being  completely  overlooked.  In  many  of  the  lower  cellular 
plants  reproduction  takes  place  by  gemmation  or  fission, 
which  may  be  continuous  or  discontinuous,  and  the  process 
differs  little  from  what  I'nay  be  observed  in  many  of  the 
lower  animals.  In  the  higiicr  plants,  however,  continuous 
gemmation  is  universal,  but  it  is  so  plainly  a  mere  form  of 
growth  that  it  is  never  regarded  as  being  of  a  reproductive 
nature.  Nevertheless,  from  a  philosophical  point  of  view, 
the  gemmation  by  which  a  tree  is  produced  may  be  in  all 
respects  paralleled  with  that  to  which  the  origin  of  one  of 
the  plant-like  colonies  of  the  Hydroid  Zoophytes  is  du.e,  if 
we  simply  make  due  allowance  for  the  differences  which 
subsist  between  animals  and  plants. 

Thus  the  leaves  of  the  tree  are  truly  "  nutritive  zooids," 
produced  by  a  process  of  continuous  ^ftnmation  from  the 
primitive  being  which  is  developed  from  the  ovum ;  and 
they  are  concerned  whoDy  with  the  nutrition  of  the  organism, 


120  ELEMENTS   OF  BIOLOGY, 

and  take  no  part  in  reproduction.  That  they  do  not  strike 
us  in  the  same  light  as  do  the  "  pol)  pites  '  of  the  Hydroid 
colony  arises  merely  from  the  fact  that  they  are  devoid  of 
th2  animal  "functions  of  relation."  In  reality,  however, 
they  lead  a  life  which  is  just  as  independent  of  the  whole, 
whilst  the  life  of  the  latter  is  in  no  way  commensurate  with 
the  existence  of  the  leaves. 

Similarly,  the  tree  ordinarily  consists  simply  of  a  collec- 
tion of  leaves,  or  nutritive  factors,  which  have  no  power  of 
producing  the  sexual  elements.  At  certain  times,  however, 
the  tree  produces  special  buds — the  floweis — in  which  the 
generative  elements  are  produced,  and  by  the  agency  of 
which  the  perpetuation  of  the  species  is  insured. 

We  may,  then,  regard  ordinary  plants  as  colonies  consist- 
ing theoretically  of  a  "trophosome"  and  "gonosome,"  each 
of  which  is  made  up  of  an  indefinite  number  of  zooids.  The 
zooids  of  the  trophosome  —  or  leaves  —  are  all  like  one 
another,  and  are  devoted  to  the  nutrition  of  the  colony. 
The  zooids  of  the  "gonosome" — or  flowers — are  also 
Msually  all  alike,  but  do  not  resemble  the  leaves,  though  the 
two  can  be  shown  to  be  morphologically  identical.  They 
take  no  part  in  the  nutrition  of  the  colony,  but  are  simply 
devoted  to  the  production  of  new  'ndividuals.  The  inter- 
esting and  important  point  about  this  comparison  is  the 
clearness  with  which  it  brings  out  the  fact  that  gemmation 
and  fission  are  merely  to  be  regarded  as  forms  of  growth. 
No  one  thinks  of  lo'^king  „pon  the  leaves  or  flowers  of  a 
tree  as  independent  or  separate  beings ;  and  yet  in  reality 
they  have  just  as  much  claim  to  this  title  as  have  the  zooids 
of  the  Hydroid  colony.  On  the  contrary,  every  one  recog- 
nises that  a  tree  is  the  result  of  a  process  of  growth  ;  and 
every  one  would  equally  recognise  that  this  is  the  case  with 
the  Hydroid?:,  if  the  polypites  of  the  latter  were  endowed  with 
as  little  power  of  spontaneous  motion  and  as  little  sensation 
as  the  leaves  of  a  plant. 

In  plants,  as  in  animais,  the  only  genuine  form  of  repro- 


REPRODUCTION   IN    PLANTS. 


121 


(luction  consists  m  the  production  of  two  cells  having  dif- 
ferent contents — a  sperm-cell  or  spermatozoid,  and  a  germ- 
cell  or  ovum.  The  contact  of  these  gives  rise  to  the  direct 
formation  of  an  embryo,  or,  in  other  cases,  to  the  formation 
of  an  individual  which  produces  special  buds  or  "  spores." 
In  all  the  higher  plants  there  is  a  male  element  or  **  pollen," 
and  a  female  element  (or  ovule),  both  cellular,  and  the  em- 
br)o  is  produced  by  the  coming  together  of  these.  In  the 
lower  plants  considerable  modifications  occur  as  to  the  man- 
ner in  which  new  individuals  are  produced  ;  but  in  the  great 
majority  of  cases  elements  corresponding  to  the  pollen  and 
ovule  of  the  higher  forms  aic  produced.  It  is  impossible 
here  to  treat  or*  the  modifications  of  the  reproductive  process 
of  plants  at  any  length  ;  but  we  may  very  briefly  describe 
the  method  by  which  new  individuals  are  produced  in  the 
ordinary  Flowering  Plants  (Angiosperms)  and  in  Ferns. 

The  male  organs  of  Angiosperm.ous  Flowering  Plants  are 
called  the  "stamens"  (fig.  35,  A),  and,  like  the  other  parts 


A  B  C 

Fig.  35- — A,  Flower  of  Tulip  with  the  external  parts  removed,  showing  the  six  sta- 
mens (.v)  surrounding  the  pistil  (/).  B,  Single  stamen  enlarged,  showing  anther 
(a)  and  the  filament  or  stalk  (/).  C,  Pollen-grains  enlarged,  one  of  them  dis- 
charging the  fovilla. 

of  the  flower-bud,  are  really  to  be  regarded  as  modified 
leaves.  Each  consists  of  a  folded  leaf  or  "anther"  (fig.  35, 
B),  which  is  generally  supported  upon  a  more  or  less  con- 


122 


ELEMENTS  OF  BIOLOGY. 


spicuous  stem  or  "  filament."  When  mature,  the  anmer  is 
found  to  be  filled  with  microscopic  cellular  bodies  or  "pol- 
len-grains" (fig.  35,  C),  which  constitute  a  fine  powdet,  and 
which  are  truly  the  male  element  of  reproduction.  The 
pollen-grains,  in  turn,  are  filled  with  an  extremely  fine 
molecular  matter  which  is  termed  the  "fovilla."  The  par- 
ticles of  the  fovilla  exhibit  more  or  less  active  movements, 
the  exact  nature  of  which  has  not  yet  been  accurately  deter- 
mined ;  and  it  is  probable  that  they  are  the  essential  gener- 
ative elements  by  which  the  influence  of  the  male  is  trans- 
mitted to  the  female. 

The  female  organs  of  Angiospermous  Flowering  Plants 
constitute  the  "  pistil "  (fig.  36,  A) ;  and  consist  in  their 


..^d 


Fig.  36. — A,  Pistil  of  the  Apricot.  B,  Pistil  of  the  Orange.  C,  Flower  of  Valerian, 
cut  vertically,  a  Ovary,  containing  the  ovule  or  ovules ;  b  Style ;  c  Stigma ; 
d  Stamen. 


simplest  and  most  fundamental  form  of  a  folded  leaf  or 
"ovary"  («),  containing  one  or  more  germ-cells  or  ovules. 
The  summit  of  the  pistil  is  formed  of  loose  cells,  which  are 
uncovered  by  epidermis,  and  secrete  a  viscid  iluid,  the 
whole  constituting  what  is  known  as  the  "  stigma  "  (c).    The 


REPRODUCTION   IN  PLANTS. 


X23 


stigma  may  be  seated  directly  upon  the  ovarv,  or  may  be 
separated  from  it  by  a  longer  or  shorter  stalk,  which  is 
termed  the  "  style  "  {b). 

The  male  and  female  organs  of  reproduction  are  usually 
present  in  the  same  flower,  when  the  plant  is  "  monoecious ;" 
but  at  other  times  one  individual  produces  the  male  flowers 
and  another  individual  produces  female  flowers,  when  the 
species  is  "  dioecious."  Even  in  bisexual  flowers,  however, 
there  is  reason  to  believe  that  there  are  natural  arrange- 
ments whereby  perpetual  self-fertilisation  is  prevented,  and 
the  influence  of  another  individual  is  at  intervals  secured. 

In  ordinary  cases  amongst  Angiosperms,  the  process  by 
which  the  ovule  is  impregnated  may  be  described  as  fol- 
lows : — The  anthers,  when  ripe,  burst,  and  shed  their  con- 
tained pollen  upon  the  moist  stigmatic  surface  of  the  pistil. 
The  viscid  secretion  of  the  stigma  seems  to  act  in  such  a 
manner  upon  the  pollen-grains  that  their  inner  lining  is 
protruded  in  the  form  of  delicate  microscopic  tubes — the 
"  pollen-tubes."  These  insinuate  their  extremities  into  the 
loose  tissue  of  the  stigma,  and,  gradually  elongating,  make 
their  way  into  the  ovary ;  the  distance  traversed  in  this 
way  varying  with^he  distance  between  the  stigma  and  ovule, 
and  being  enormously  great  in  long-styled  plants.  During 
this  process,  changes  have  been  going  on  in  the  ovule,  in 
consequence  of  which  impregnation  is  possible.  The  most 
important  of  these  consists  in  the  enlargement  of  the  so- 
called  "  embryo-sac,"  which  truly  corresponds  with  the 
ovum  of  animals,  and  the  formation  in  its  interior  of  from 
one  to  three  or  more  vesicular  bodies,  which  are  known  as 
the  "  embryonal  vesicles,"  and  which  seem  to  correspond 
with  the  germinal  vesicle  of  the  ovum  of  animals.  When 
the  pollen-tube  reaches  the  embryo-sac,  its  further  growth 
seems  to  be  generally  arrested,  and  it  is  only  in  rare  cases 
that  the  pollen-tube  perforates  the  embryo-sac,*  if,  indeed, 

*  Recent  researches  demonstrating  the  possilj|Iity  of  cells  making 
their  way  through  unbroken  surfaces,  as  has  been  incontestably  proved 


124  ELEMENTS  OF  BIOLOGY. 

this  ever  really  happens.  The  fluid  matter  of  the  pollen- 
tube,  and  possibly  some  of  the  minutely  granular  "  fovilla  " 
as  well,  is  now  transferred  to  the  embryo-sac  j  and  as  the 
result  of  the  stimulus  thus  imparted,  one  or  more  of  the 
embryonal  vesicles  is  impregnat  1,  when  the  pollen-tubes 
decay. 

As  regards  the  reproduction  of  the  Flowerless  Plants 
{Cryptogams),  the  process  varies  much  in  different  cases; 
but  in  th'e  higher  forms  the  essential  element  of  the  process 
consists  in  the  production  of  sperm-cells  or  spermatozoa,  and 
a  germ-cell  or  ovum.  There  are,  however,  some  very  singu- 
lar complexities  in  the  manner  in  which  these  essential 
generative  elements  are  produced,  and  we  may  notice  the 
phenomena  which  have  been  observed  in  Ferns  : — 

The  ordinary  Ferns  are  well  known  to  produce  at  certain 
seasons  what  are  commonly  spoken  of  as  the  "organs  of 
fructification."  In  the  commoner  species  these  take  the 
form  of  little  rounded  masses,  which  are  generally  placed 
upon  the  back  of  the  adult  frond  (fig.  37,  A).  When 
examined  microscopically,  each  of  these  spots  of  fructifica- 
tion is  found  to  consist  of  an  aggregation  of  minute  recep- 
tacles or  "  spore-cases,"  containing  in  fheir  interior  still 
more  minute  cellular  bodies  or  "  spores."  If  one  of  these 
spores  be  liberated  from  the  spore-case,  and  placed  under 
favourable  conditions,  it  germinates,  giving  off  roots  on  the 
one  hand,  and  producing  on  the  other  hand  a  little  cellular 
expansion  or  leaf,  which  is  termed  the  "  prothallus  "  (fig. 
37,  D).  This  prothallus,  however,  is  not  itself  developed 
into  a  new  fern,  but  it  is  a  mere  temporary  or  provisional 
body,  upon  which  are  produced  male  and  female  organs 
of  reproduction.  The  male  organs  are  produced  upon  the 
under  side  of  the  prothallus,  and  they  have  the  form  of 
minute  cellular  eminences,  containing   reproductive   cells. 

in  the  case  of  the  white  corpuscles  of  the  blood,  render  it  by  no  means 
unlikely  that  the  fovilla  itself  reaches  the  embryo-sac  without  any  neces- 
sary rupture  of  the  walls  of  this  cavity  or  of  the  pollen-tube. 


REPRODUCTION  IN    PLANTS. 


125 


These  cells  are  liberated,  when  they  burst,  and  give  exit  to 
true  spermatozoa  in  the  form  of  ciliated  spiral  filaments. 
The  female  organs  are  also  placed  upon  the  under  surface 
of  the  prothallus,  and  also  have  the  form  of  cellular  pro- 


Fig.  37. — A,  Portion  of  the  frond  of  PolyPodiuvi  viilgare,  showing  the  organs  01 
fructification  B,  Spore-cases  of  the  same,  magnified.  C,  Sporeof  a  Fern,  greatly 
enlarged.  D,  Cellular  prothallus  of  a  Fern,  produced  by  a  spore  {s),  and  giving 
off  a  root  (r). 

minences.  The  cells  of  these  prominences  are  so  arranged 
that  they  form  a  canal,  leading  down  to  a  large  central  cell 
or  ovule. 

The  spermatozoa  liberated  from  the  male  organs  pass 
down  the  central  canal,  and  gain  access  to  the  ovule. 
As  the  result  of  this,  segmentation  of  the  ovule  is  set  up, 
and  an  embryo  is  produced  from  which  the  frond  of  the 
ordinary  fern  is  developed,  the  prothallus  perishing  when 
this  is  accomplished. 

The  sequence  of  phenomena  here  indicated  may  in  some 
respects  be  fairly  compared  with  those  formerly  alluded  to 
under  the  head  of  "  alternation  of  generations."  The 
"  spores  "  produced  in  the  spore-cases  of  the  ordinary  fern 
are  to  be  regarded  simply  as  buds,  since  they  are  not  pro- 
duced by  any  generative  act,  whilst  they  have  the  power  of 
develo^ng  themselves  without  contact  with  a  second  dis- 


126 


ELEMENTS  OF  BIOLOGY. 


similar  element.  These  spores  give  rise  to  a  temporary 
organism,  the  sole  function  of  which  is  to  develop  the 
special  organs  in  which  the  essential  elements  of  reproduc- 
tion may  be  produced.  The  contact  of  these  elements  gives 
rise  to  an  embryo,  which  is  developed  into  the  original 
"  sporangiferous "  frond  by  which  the  spores  were  pro- 
duced, and  not  into  the  temporary  cellular  expansion  on 
which  the  generative  elements  were  carried.  There  is  thus 
an  alternation  of  gemmation  with  generation,  the  generative 
process  being  carried  on  by  a  minute  provisional  organism, 
developed  by  budding  from  a  conspicuous  leafy  frond, 
which  latter  is  produced  in  a  true  sexual  manner. 


>^ 


■'     ;i  1  r 


(  '  ''  ;  .  I 


'  V, 


"V 


CHAPTER    XII. 


SPONTANEOUS   GENERATION. 


"Spontaneous  Generation,"  or  " Abiogenesis,"  is  the 
term  applied  to  the  alleged  production  of  living  beings 
without  the  pre-existence  of  germs  of  any  kind,  and  there- 
fore without  the  pre-existence  of  parent  organisms.  -  The 
question  as  to  the  possibility  of  spontaneous  generation  is 
one  which  has  been  long  and  closely  disputed,  and  which 
cannot  be  said  to  be  yet  definitely  settled.  It  will  be  suffi- 
cient, therefore,  to  indicate  some  of  the  facts  upon  which 
the  belief  in  Abiogenesis  is  founded,  and  to  point  out  some 
general  considerations  upon  the  same. 

If  an  animal  or  vegetable  substance  be  soaked  in  hot  or 
cold  water,  we  obtain  what  is  called  an  "  organic  infusion  " 
— that  is  to  say,  a  fluid  holding  organic  matter  in  solution. 
If  such  an  infusion  be  boiled,  any  adult  living  beings  which 
might  be  present  in  it  are  destroyed,  and  the  fluid  certainly 
becomes  temporarily  deprived  of  all.  active  life.  If,  how- 
ever, such  an  infusion  be  exposed  for  a  certain  length  of 
time  to  the  air,  a  series  "of  changes  is  inaugurated  which  end 
in  its  becoming  tenanted  by  numerous  living  organisms. 

The  first  phenomenon  observable  is  usually  the  forma- 
tion upon  the  surface  of  the  infusion  of  a  delicate  film  or 
scum.  If  a  fragment  of  this  film  or  pellicle  be  examined 
microscopically,  it  is  found  to  consist  of  numberless  moving 


128 


ELEMKNTS  OF  BIOLOGY. 


points,  particles,  or  molecules  (tig.  38,  A).  The  largest  of 
these  may  not  be  more  than  one  ten-thousandth  of  an  inch 
in  diameter;  the  smallest  may  not  exceed  one  forty-thou- 
sandth of  an   inch.       Every  increase   in   the   magnifying 


Fig.  38. — A,  Living  p.irticles  or 
15,  Bacteria  developed  in 


iSSti^i    Ck 


B 

molecules  developed  in  organic  infusions, 
organic  infusions.      {After  Beale.) 


power  of  the  microscope  has  simply  served  to  bring  to 
light  myriads  of  smaller  and  smaller  particles ;  and  the 
highest  powers  of  the  microscope  known  to  us — enormous 
as  they  are — only  leave  us  in  the  certainty  that  if  we  could 
obtain  still  higher  powers,  we  should  almost  infallibly  dis- 
cover particles  still  more  minute.  All  the  particles  of  the 
scum  are  seen  to  be  in  active  and  incessant  movement,  and 
there  is  no  question  as  to  their  being  truly  living  organisms, 
though  it  is  uncertain  whether  they  are  of  an  animal  or 
vegetable  nature,  or  whether  they  may  not  be  partly  the 
one  and  partly  the  other.      .^     -^   <    ?     .     . 

If  the  fluid  be  examined  at  a  later  period,  in  addition  to 
the  minuter  moving  particles,  there  will  be  found  many  little 
moving  filaments  of  a  larger  size.  Some  of  these  are  short 
and  staff-shaped,  and  are  known  as  "bacteria"  (fig.  38,  B). 
Others  are  long  and  worm-like,  and  move  about  actively, 
twisting  from  side  to  side.  These  are  known  as  "  vibrios." 
Both  the  bacteria  and  vibrios  are  unquestionably  alive, 
though  in  this  case,  also,  it  is  a  matter  of  some  doubt 
whether  we  have  to  deal  with  animal  or  vegetable  organ- 
isms.    Upon  the  whole,  however,  it  seems  tolerably  certain 


SPONTANEOUS  GENKKATION. 


129 


that  the  bacteria  and  vibrionos  are  fo  be  regarded  as  be- 
longing to  the  vegetable  kingdom. 

Lastly,  at  a  still  later  period,  the  fluid  may  be  found  to 
contain  forms  of  the  so-called  "  Infusorian  Animalcules." 
These  are  undoubted  animals,  and  though  not  standing 
very  high  in  the  zoological  scale,  they  are  by  no  means  the 
humblest  or  most  lowly  organised  members  of  the  animal 
kingdom. 

The  phenomena  just  recounted  are  altogether  beyond 
doubt,  and  may  be  observed  by  any  one  for  himself  with  a 
little  trouble  and  a  tolerably  good  microscope.  Their  ex- 
planation, however,  has  been  the  subject  of  one  of  the 
most  vigorous  controversies  which  has  ever  divided  the 
scientific  world  into  two  opposing  camps  ;  and  it  cannot 
be  regarded  as  by  any  means  near  its  final  settlement.  The 
point  to  be  settled  is  this  : — How  does  a  fluid  which,  to 
begin  with,  is  wholly  without  living  beings,  become  the 
home  of  unquestionable  living  organisms  ?  Two  answers 
have  been  given  to  this  question.  The  oldest  theory,  and 
one  which  was  in  vogue  long  anterior  to.  the  discovery  of 
the  facts  just  mentioned,  was,  that  these  living  beings  formed 
themselves  spontaneously  and  de  novo  out  of  the  dead 
materials  of  the  fluid.  Very  ancient  is  this  belief,  that 
living  beings  could  be  produced  by  the  spontaneous  action 
of  a  genial  and  prolific  nature  upon  dead  matter ;  and 
many  animals,  both  real  and  imaginary,  have  been  asserted 
to  have  been  generated  in  this  fashion.  Nowadays,  how- 
ever, the  theory  of  spontaneous  generation  has  been  wholly 
given  up  as  regards  all  the  cases  in  which  the  ancients  placed 
#credence  ;  and  it  has  become  entirely  restricted  to  a  group 
of  minute  organisms,  the  very  existence  of  which  has  only 
been  known  since  the  microscope  has  reached  something 
like  its  present  perfection.  Stated  briefly,  then,  as  far  as 
concerns  the  facts  above  described,  it  is  held  by  one  school 
that  the  microscopic  organisms  which  make  their  appear- 
ance in  organic  infusions,  after  exposure  to  the  air,  have 


130  ELEMENTS  OK  UIOLOGY. 

been  produced  spontaneously  by  the  action  of  physical  and 
chemical  forces  upon  the  organic,  but  dead,  materials  held 
in  solution  in  the  fluid. 

By  another  school,  on  the  other  hand,  it  is  held  that  the 
facts  of  the  case  may  be  explained  upon  the  supposition 
that  the  air,  all  fluids  exposed  to  the  atmosphere,  and 
many  solid  bodies,  are  crowded  with  the  microscopic 
germs  of  minute  living  beings,  animal  or  vegetable ;  that 
these  germs  may  remain  dormant  for  indefinite  periods, 
having  the  power  of  withstanding  temperatures  which  would 
be  fatal  to  adult  organisms  ;  but  that  they  spring  into  active 
life  the  moment  the  conditions  which  surround  them  are 
favourable  for  their  development.  Such  conditions  are 
presented  by  any  fluid  holding  organic  matter  in  solution  ; 
and  it  is  believed  that  the  living  organisms  which  appear  in 
an  organic  infusion  are  merely  developed  from  inconceiv- 
ably minute  germs,  which  fall  into  the  infusion  from  the  air, 
or  are  contained  in  ihe  fluid  to  begin  with. 

It  must  be  admitted  that  ihe  above  is  to  a  certain  extent 
an  hypothesis ;  but  it  is  not  only  supported  by  various  ab- 
stract considerations  of  great  weight,  but  also  rests  upon  a 
firm  if  somewhat  narrow  basis  of  fact.  Thus  it  has  been 
shown,  beyond  a  question,  that  such  germs  are  present  in 
the  atmosphere,  and  in  many  other  localities  as  well.  We 
may  therefore  safely  assume  as  proved,  that  the  air,  most 
fluids,  and  many  organic  and  inorganic  substances,  contain 
the  germs  of  organisms  which  are  capable  of  being  developed 
in  active  life,  when  once  they  are  placed  under  suitable  con- 
ditions. We  may  regard  this  as  proved  wholly  irrespective 
of  the  belief  that  certain  low  organisms  can  be  produced* 
spontaneously,  without  the  presence  of  pre-existent  germs. 
Even  if  spontaneous  generation  were  proved  to  be  part  of 
the  order  of  nature,  the  importance  or  validity  of  the  fact 
just  stated  would  be  in  no  way  affected  thereby.  Even  if 
we  were  to  admit  the  possible  formation  of  living  beings  out 
of  dead  matter,  it  would  still  remain  certain  that  all  nature 


SPONTANEOUS  (iENERATION.  I3I 

teems  with  a  life  invisible  except  to  the  higher  powers  of  the 
microscope — a  life  which  reproduces  itself  by  the  ordinary 
and  natural  methods,  and  which  is  ever  on  the  alert  to  catch 
the  first  opportunity  of  spiinging  into  active  instead  of  po- 
tential vitality.  ' 

It  is  not  necessary  here  to  enter  upon  the  experimental 
evidence  upon  this  subject.  Upon  no  single  question,  pro- 
bably, in  the  whole  range  of  Biology,  have  greater  pains  been 
expended,  and  more  elaborate  experiments  carried  out ;  and 
upon  no  single  question  have  the  actual  results  of  the  incjuiry 
been  so  singularly  contradictory  and  unsatisfactory.  All  the 
experiments  which  have  been  set  on  foot  with  a  view  of 
settling  this  question  have  been  directed  to  one  of  two  ends 
— viz.,  to  prove  that  no  life  would  appear  in  organic  infu- 
sions from  which  germs  were  rigidly  excluded,  or  to  show 
that  living  organisms  appeared  in  fluids  in  which  it  was  im- 
possible that  any  germs  could  be  present.  Neither  end  has 
as  yet  been  satisfactorily  attained ;  and  from  the  nature  of 
the  case  't  is  difficult  to  believe  that  the  experimental  evi- 
dence could,  under  any  circumstances,  ever  amount  to 
actual  demonstration.  For  our  present  purpose  it  will  be 
sufficient,  very  briefly,  to  consider  some  recent  experiments 
carried  out  by  Dr  Charlton  Bastian  with  a  view  of  proving 
the  occurrence  of  Abiogenesis.    ;."*'<■     (..';/.'  .r 

The  most  important  experiments  carried  out  bjr  this  ob- 
server consisted  in  taking  an  organic  infusion — such  as  an 
infusion  of  turnip — boiling  it,  to  expel  the  air  as  far  as  pos- 
sible, as  well  as  to  kill  any  germs  which  might  be  present  in 
the  fluid,  and  then  hermetically  sealing  the  neck  of  the  flask 
in  the  flame  of  a  spirit-lamp.  By  this  procedure  it  will  be 
at  once  evident  that  the  experimenter  had  an  infusion  con- 
taining dead  organic  matter,  but  ostensibly  containing  no 
living  germs,  enclosed  in  a  flask  from  which  all,  or  nearly 
all,  the  atmospheric  air  had  been  expelled  by  boiling.  The 
flask  thus  prepared  was  submitted  for  hours  to  a  tempera- 
ture considerably  over  the  boiling-point  of  water,  and  then 


132  KLLMENIS   UV   iilOl.OGY. 

allowed  to  remain  unopened  for  a  varying  period.  It  only 
remains  to  add,  that  in  some  of  the  experiments  the  rigour 
of  the  conditions  was  still  further  increased  by  the  substitu- 
tion for  the  organic  infusion  of  mere  solutions  of  certain 
salts,  such  as  tartar  emetic,  phosphate  of  ammonia,  or  phos- 
phate of  soda. 

With  regard  to  the  alleged  results  of  these  apparently 
crucial  experiments,  Dr  Bastian  asserts  that  in  almost  every 
instance  the  fluid  in  the  flask,  in  the  course  of  a  certain 
time,  was  found  under  the  microscope  to  exhibit  numerous 
living  organisms,  chiefly,  though  not  exclusively,  of  a  vege- 
table nature. 

With  regard  to  the  value  of  these  results,  it  should  be 
remarked,  in  the  first  place,  that  the  conditions  of  the  ex- 
periment were  such  as  we  should,  upon  a  priori  grounds, 
have  believed  to  be  utterly  fatal  to  the  possibility  of  the 
development  of  life  even  in  its  humblest  forms.  The  fluid 
experimented  on  was  subjected  to  a  temperature  exceeding 
that  of  boiling  water,  and  the  flasks  were  hermetically  sealed 
at  a  time  when  they  were  filled  with  steam,  so  that  the 
atmospheric  air  was  thereby  excluded  from  them.  It  is  true 
that  the  experiments  of  Pasteur  have  shown  that  some  of 
the  organisms  of  infusions — e.g.^  the  bacteria — can  exist 
without  free  oxygen ;  but  there  is  certainly  no  reason  to 
believe  t^at  any  living  beings  can  thrive  in  a  complete  and 
perfect  vacuum.  In  the  second  place,  it  is  to  be  noticed 
that  in  spite  of  the  fearfully  deterrent  conditions  under  which 
the  fluid  in  the  flasks  was  placed,  living  beings  are  alleged 
to  have  made  their  appearance  therein  nearly  or  quite 
as  abundantly  as  would  have  been  the  case  if  an  ordinary 
organic  infusion  had  been  taken,  subjected  to  ordinary  con- 
ditions, and  allowed  an  unrestrained  access  of  air. 

In  the  third  place,  there  is  absolutely  no  proof  that  the 
heat  to  which  the  fluids  experimented  on  were  subjected  is 
sufficient  to  kill  any  or  all  living  germs.  It  is  quite  true 
that,  so  far  as  we  know,  no  adult  organism  can  withstand 


SPONTANEOUS  GENERATION. 


133 


for  any  length  of  time  exposure  to  a  temperature  equal  to 
that  of  boiling  water.  Ihit  it  by  no  means  follows  from  this 
that  the  same  temperature  would  necessarily  suffice  to  de- 
stroy all  the  indescribably  minute  germs  from  which  some 
of  the  lower  animals  and  plants  are  produced.  In  point  of 
fact,  many  instances  are  known  in  which  the  eggs  of  various 
animals,  and  the  seeds  of  many  plants,  can  withstand  in- 
jurious conditions  to  an  extent  of  which  the  adults  are 
wholly  incapable.  And  Mr  Grace  -  Calvert  has  recently 
shown  that  vibrios  can  endure  a  temperature  in  some  cases 
exceeding  300°  Fahr.  without  being  killed  thereby. 

Lastly,  many  of  the  vegetable  organisms  present  in  the 
infusions  of  Dr  Bastian  were  seen  fructifying  and  producing 
spores  in  the  ordinary  manner.  Had  they  b^cn  produced 
spontaneously,  and  had  this  mode  of  production  been  the 
natural  one,  it  would,  to  say  the  least  of  it,  be  a  very  remark- 
able fact  if  they  should  straightway  proceed  to  reproduce 
their  kind  in  the  manner  which  is  believed  to  be  the  normal 
and  regular  mode.  This  argument  is  a  still  stronger  one 
when  applied  to  the  Infusorian  Animalcules,  which  are  so 
commonly  found  -in  organic  infusions,  but  which  do  not 
appear  to  have  ma  "  their  appearance  in  any  of  the  fluids 
experimented  on  by  Dr  Bastian.  In  this  case,  not  only  is 
the  organisation  of  the  animals  of  a  comparatively  high 
type,  but  we  are  perfectly  familiar  with  their  modes  of  re- 
production ;  and  it  would  appear  to  be  most  unnecessary 
that  they  should  be  produced  spontaneously  in  the  manner 
alleged,  since  their  fecundity  by  the  ordinary  methods  of 
reproduction  is  very  great. 

Upon  the  whole,  then,  we  can  hardly  avoid  the  conclu- 
sion that  some  fallacy  lurks  under  the  experiments  carried 
out  by  Dr  Bastian.  Probably  the  Uving  germs  of  the  low- 
est animals  and  plants  are  not  destroyed  by  a  temperature 
equal  to  that  of  boiling  water;  whilst  some  of  the  lower 
forms  CA  life  may  be  able  to  endure  conditions  which  might 
at  first  sight  be  regarded  as  inevitably  destructive  of  vitality. 
7 


CHAPTER     XIII. 


ORIGIN   OF    SPECIES. 


We  have  already  seen  reasons  to  conclude  that  the  term 
"  species "  must  be  regarded  as  being  merely  a  convenient 
abstraction  by  which  we  denote  assemblages  of  individuals 
having  certain  characters  in  common.  We  are  thus  led  to 
the  belief  that  what  naturalists  ordinarily  call  "  species  "  are 
not  unvarying  and  immutable  quantities.  We  cannot,  there- 
fore, retain,  in  the  sense  in  which  he  used  it,  the  dictum  of 
Forbes,  that  "  every  true  species  presents '  in  its  individuals 
certain  features,  specific  characters,  which  distinguish  it 
from  every  other  species ;  as  if  the  Creator  had  set  an  exclu- 
sive mark  or  seal  on  each  type."  On  the  contrary,  the 
researches  of  Darwin,  Wallace,  and  others,  have  compelled 
the  admission  that  all  so-called  species  vary  more  or  less 
and  that  these  variations  are  sometimes  so  extensive  that 
the  limits  of  specific  distinctness  are  overstepped.  Still,  it 
has  not  yc";  been  demonstrated  that  these  variations  are 
indeLnite,  either  in  direction  or  amount;  and  it  remains, 
therefore,  possible  that  the  process  of  specific  variation  is 
bounded  by  fixed,  if  widely  extended,  limits,  however  pro- 
bable the  contrary  may  appear. 

It  is  impossible  here  to  do  more  than  merely  indicate,  in 
the  briefest  manner,  the  two  fundamental  ideas  which  are  at 
the  bottom  of  the  leading  theories  which  are  entertained  as 


ORIGIN  OF  SPECIES. 


135 


to  the  origin  of  species.  The  opinions  of  scientific  men  are 
still  divided  upon  this  subject ;  and  it  will  be  sufficient  to 
give  an  outline  of  the  two  more  important  hypotheses,  with- 
out adducing  any  of  the  reasoning  upon  which  they  are 
based,* 

I.  Doctrine  of  Special  Creation. — Upon  this  doctrine 
of  the  origin  of  species,  it  is  believed  that  species  are  to  all 
practical  intents  and  purposes  immutable  productions,  each 
of  which  has  been  specially  created  at  some  point  within  the 
area  in  which  we  now  find  it,  subsequently  spreading  from 
this  spot  as  far  as  the  conditions  of  life  were  suitable  for  it. 
Each  species  upon  this  view  has  a  "  specific  centre,"  where 
it  was  primitively  created,  and  from  which  it  extended  itself 
over  a  larger  or  smaller  area,  until  its  progress  was  stopped 
by  unsuitable  conditions.  Upon  this  theory,  therefore,  if  a 
species  is  found  occupying  two  widely  remote  areas,  this 
can  only  be  in  consequence  of  some  geological  change 
by  which  the  original  area  became  divided,  or  in  conse- 
quence of  the  species  having  been  carried  in  some  acci- 
dental manner  to  a  considerable  distance  from  its  original 
home. 

II.  Doctrine  of  Evolution. — On  the  other  hand,  it  is 
believed  that  species  are  not  permanent  and  immutable,  but 
that  they  "  undergo  modification,  and  that  the  existing  forms 
of  life  are  the  descendants  by  true  generation  of  pre-existing 
forms  "  (Darwin).  Upon  this  view  the  resemblances  which 
we  express  by  the  terms  species,  genus,  family,  order,  and 
the  like,  indicate  really  the  existence  of  a  true  blood-relation- 
ship between  the  organisms  thus  grouped  together,  each 
group  denoting  a  less  and  less  close  degree  of  relationship 
as  we  recede  from  the  "species"  in  the  direction  of  the 
"sub-kingdom."  Whilst  most  naturalists  are  inclined  to 
admit  the  truth  of  the  general  doctrine  of  Evolution,  as 

*  The  author  would  ask  his  readers  to  remember  that  the  mere  state- 
ment of  the  leading  propositions  of  two  ppposing  theories  in  no  way 
commits  the  writer  to  the  support  or  rejection  of  either. 


136  ELEMENTS  OF  BIOLOGY. 

expressed  in  the  above  proposition,  considerable  difference 
of  opinion  obtains  as  to  the  method  in  which  evohition  has 
been  brought  about. 

On  Lamarck's  theory  of  the  evolution  of  species,  the 
means  of  modification  were  ascribed  to  the  action  of  exter- 
nal physical  agencies,  the  interbreeding  of  already  existing 
forms,  and  the  effects  of  habit,  or  the  use  and  disuse  of  cer- 
tain organs. 

The  doctrine  of  the  evolution  of  species  by  variation  and 
"  Natural  Selectior; " — propounded  by  Mr  Darwin,  and  com- 
monly known  as  the  Darwinian  theory — is  based  upon  the 
following  fundamental  propositions  : — 

1.  The  progeny  of  all  species  of  animals  and  plants 
exhibit  variations  amongst  themselves  in  all  parts  of  their 
organisation,  no  two  individuals  being  exactly  and  in  all 
respects  alike.  In  other  words,  in  every  species  the  indi- 
viduals, whilst  inheriting  a  general  likeness  to  their  progeni- 
tors, tend  by  variation  to  diverge  from  the  parent  type  in 
some  particular  or  other. 

2.  Variations  arising  in  any  part  of  the  organism,  how- 
ever minute,  may  be  transmitted  to  future  generations,  under 
certain  definite  and  discoverable  laws  of  inheritance. 

3.  By  "artificial  selection,"  or  by  breeding  from  indi- 
viduals possessing  any  particular  variation,  man,  in  succes- 
sive generations,  can  produce  a  breed  in  which  the  variation 
will  be  permanent,  the  divergence  from  the  parent  type 
being  usually  intensified  by  the  process  of  interbreeding. 
The  races  thus  artificially  produced  by  men  are  often  as 
widely  different  as  are  distinct  species  of  wild  animals. 

4.  The  world  in  which  all  living  beings  are  placed  is  one 
not  absolutely  unchanging,  but  is  liable,  on  the  contrary,  to 
subject  them  to  very  varying  conditions. 

5.  All  animals  and  plants  give  rise  to  more  numerous 
young  than  can  by  any  possibility  be  preserved,  each  spe- 
cies tending  to  increase  in  numbers  in  a  geometrical 
progression. 


ORIGIN  OF  SPECIES.  I37 

6.  As  these  young  are  none  of  them  exactly  alike  in  all 
respects,  a  pfbcess  of  "  Natural  Selection  "  will  ensue,  where- 
by those  individuals  which  possess  any  variation,  however 
slight,  favourable  to  the  peculiarities  of  the  species,  will  tend 
to  be  preserved.  Those  individuals,  on  the  other  hand, 
which  do  not  possess  any  such  favourable  variation,  will  be 
placed  at  a  disadvantage  in  the  "  struggle  for  existence,"  and 
will  tend  to  be  gradually  exterminated.  The  individuals, 
therefore,  composing  any  species,  are  thus  subjected  to  a 
rigid  process  of  sifting,  by  which  those  least  adapted  to 
their  environment  are  being  perpetually  weeded  out,  whilst 
'*  the  survival  of  the  fittest "  is  secured. 

7.  Other  conditions  remaining  the  same,  the  individuals 
which  survive  in  the  struggle  for  existence  will  transmit  the 
variations,  to  which  they  owe  their  preservation,  to  future 
generations. 

8.  By  a  repetition  of  this  process,  "  varieties  "  are  first 
established ;  these  become  permanent,  and  "  races  "  are 
produced ;  finally,  in  the  lapse  of  time,  the  differences  thus 
caused  become  sufficiently  marked  to  constitute  distinct 
"  species." 

9.  If  we  grant  that  past  time  has  been  practically  infinite, 
it  is  conceivable  that  all  the  different  animals  and  plants 
which  we  see  at  present  upon  the  globe,  may  have  been 
produced  by  the  action  of  Natural  Selection  upon  the  off- 
spring of  a  few  primordial  forms,  or,  it  may  be,  of  a  single 
primitive  being. 

Originally,  Mr  Darwin  appears  to  have  believed  that 
"Natural  Selection"  would  alone  be  found  to  be  a  suffi- 
cient cause  to  have  given  rise  to  all  existing  species  by  a 
process  of  Evolution  from  pre-existing  forms.  In  view, 
however,  of  certain  objections  which  had  been  brought  for- 
ward, Mr  Darwin  seems  to  have  abandoned  this  position ; 
and  a  cause  supplementary  to  "  Natural  Selection "  was 
sought  for  in  what  Mr  Darwin  terms  "  Sexual  Selection." 
The  action  of  Sexual  Selection  in  a  supposed  process  of 


138  ELEMENTS  OF  BIOLOGY. 

Evolution,  according  to  Mr  Darwin's  views,  may  be  stated 
in  the  following  two  propositions  : —  • 

a.  The  males  of  man;'  species  of  animals  are  known  to 
engage  in  very  severe  contests  for  the  possession  of  the 
females,  these  latter  yielding  themselves  to  the  victor.  In 
such  contists  certain  males  will  inevitably  have  certain  ad- 
vantages over  the  others,  either  in  point  of  strength  or  ac- 
tivity, or  in  consequence  of  the  possession  of  more  efficient 
offensive  weapons.  There  will  therefore  always  be  a  pro- 
bability that  certain  males  will  get  possession  of  the  females 
in  preference  to  others ;  and  thus  there  will  be  a  tendency 
in  the  individuals  of  many  species  of  animals  to  secure  a 
preponderance  of  offspring  from  the  strongest  males.  The 
peculiarities  which  enable  certain  males  to  succeed  in  these 
contests  will,  ceteris  paribus^  be  transmitted  to  their  male 
offspring,  and  in  this  way  variations  may  be  perpetuated, 
initiated,  or  intensified. 

b.  In  the  preceding  cases,  the  females  are  believed  to  be 
perfectly  passive,  and  the  selection  is  a  "  natural "  one,  the 
final  result  depending  solely  upon  the  natural  advantages 
which  certain  males  possess  over  others  in  actual  combat. 
It  is  alleged,  however,  that  there  are  other  cases  in  which 
the  selection  is  truly  "  sexual,"  since  its  result  is  determined 
by  spontaneous  preference,  and  not  by  brute  force  alone. 
It  is  asserted,  namely,  that  amongst  certain  species  of  ani- 
mals, the  females  exercise  a  free  choice  as  to  the  particular 
male  with  which  they  will  pair;  the  males  being  passive 
agents  in  the  matter,  except  in  so  far  as  each  uses,  or  may 
use,  his  utmost  exertions  to  secure  that  the  choice  of  the 
female  may  fall  upon  him.  The  circumstances  supposed 
to  influence,  and  ultimately  determine,  the  choice  of  the 
female,  are  of  course,  in  the  main,  the  personal  attractions 
of  some  particular  male,  the  female  being  captivated  by 
some  "  beauty  of  form,  colour,  odour,  or  voice,"  which  such 
a  male  may  possess. 

If  it  be  admitted  that  the  females  of  some  of  the  lower 


ORIGIN  OF  SPECIES.  1 39 

animals  have  the  power  of  expressing  and  exercising  a  pre- 
ference in  the  manner  above  indicated,  then  it  is  easy  to 
understand  how  variations  might  be  transmitted  or  intensi- 
fied in  this  way.  The  male  who  is  most  attractive  to  the 
female,  will,  other  things  being  equal,  have  the  best  chajice 
of  propagating  his  species,  and  is  likely  to  leave  the  largest 
number  of  descendants.  His  male  offspring  will  inherit  the 
peculiarities  by  which  theif  sire  was  rendered  pre-eminently 
attractive  in  the  eyes  of  their  mother,  and  thus  a  well-marked 
breed  might  be  produced,  by  the  preservation  or  intensifi- 
cation of  characters  of  this  nature.  Mr  Darwin  is  disposed 
to  believe  that  colour  .ind  song  in  most,  if  not  in  all,  animals 
are  thus  to  be  ascribed  to  the  action  of  Sexual  Selection, 
through  numerous  successive  generations ;  but  other  com- 
jietent  authorities  are  unable  to  concur  in  this  view. 

Numerous  objections  have  been  brought  fonvard  to  pro/e 
the  insufficiency  of  the  view  that  the  Evolution  of  species 
has  been  effected  by  Natural  Selection.  The  student  de- 
sirous of  making  himself  acquainted  with  this  subject  should 
consult  Mr  Mivart's  '  Genesis  of  Species ; '  but  the  follow- 
ing are  the  chief  difficulties  which  the  advocate  of  Natural 
Selection  has  to  meet : — 

I.  Natural  Selection,  whilst  doubtless  capable  of  preserv- 
ing favourable  variations,  cannot  initiate  changes  of  any 
kind.  The  origin,  therefore,  of  variations  is  not  elucidated 
in  any  vay  by  the  doctrine  of  Natural  Selection, and  we  are 
coi.-^'^i  ed  to  believe  that  the  variability  of  the  individuals 
of  a  species  depends  upon  some  internal  law  with  which  we 
are  not  as  yet  acquainted.  It  thus  remains  open  for  us  to 
believe  that  the  law  which  gives  rise  to  variations  is  in 
every  way  a  more  important  one  than  that  under  which 
they  are  simply  preserved.  Unfavourable  variations  must 
be  at  least  as  common  as  those  which  are  advantageous, 
and  whilst  Natural  Selection  can  produce  neither,  it  can  at 
best  \ycX preserve  the  latter.  It  seems  clear  also  that  many 
variations  which,  when  fully  developed,  are  very  useful  to 


140  ELEMENTS   OF  BIOLOGY. 

the  species,  would,  to  begin  with,  be  so  minute  as  to  be  use- 
less, if  not  injurious,  in  which  case  their  preservation  and 
ultimate  intensification  must  have  been  caused  by  something 
else  than  Natural  Selection  alone. 

2.  Whilst  Natural  Selection  cannot  initiate  even  the 
smallest  variations,  the  belief  in  its  being  a  constant  and 
universal  agent  in  modifying  all  living  beings,  requires  that 
variations  should  be  continually  occurring,  and  that  they 
should  not  be  extensive  in  amount.  The  probability,  how- 
ever, that  all  variations  depend  upon  some  internal  law  far 
below  the  surface,  and  unconnected  with  outside  conditions, 
is  greatly  increased  by  the  undoubted  occurrence  of  sudden 
and  striking  variations,  for  which  no  cause  can  be  shown, 
and  for  which  Natural  Selection  is  unable  to  account. 

3.  It  has  been  shown  that  it  is  not  sufficient  for  the  pro- 
duction of  a  new  breed  or  variety  simply  that  a  favourable 
variation  should  occur,  unless  the  change  should  occur 
simultaneously  in  a  greater  or  less  number  of  individuals 
of  the  species.  However  favourable  a  variation  might  be, 
there  would  be  little  or  no  chance  of  its  being  perpetuated, 
unless  it  presented  itself  in  more  than  one  individual  at  the 
same  time.  But  the  probabilities  are  enormously  against 
the  simultaneous  appearance  of  the  same  variation  in  numer- 
ous individuals  of  a  species.  We  are  thus  led  to  doubt  if 
even  highly  favourable  variations  would  necessarily,  or  even 
probably,  end  in  the  establishment  through  natural  selection 
of  a  permanent  new  breed  or  variety. 

4.  The  same  parents  may  give  rise  to  several  groups  of 
individuals  which  differ  very  widely  from  one  another,  and 
from  their  parents  in  their  characters,  but  which  are  sexless, 
and  are  therefore  unable  to  transmit  their  peculiarities  to 
future  generations.  Thus  the  workers  and  the  soldiers 
amongst  the  Termites  differ  greatly  both  from  one  another 
and  from  the  fertile  individuals,  both  in  their  actual  struc- 
ture and  in  their  instincts ;  and  yet  both  are  neuter  and 
have  no  power  of  transmitting  their  peculiarities  by  the 


ORIGIN   OF   SPECIES.  141 

way  of  inheritance.     Yet  it  is  only  by  the  medium  of  her- 
edity that  Natural  Selection  can  possibly  act. 

5.  Whilst  it  is  undeniable  that  the  individuals  composing 
any  species  vary  more  or  less  amongst  themselves,  there  is 
no  proof  that  the  variability  of  any  species  is  itidcfinite.  On 
the  contrary,  there  are  reasons  to  believe  that  each  species 
is  bounded  by  an  uncertain  but  definite  range  of  variability. 
The  extreme  terms  of  this  range  may  lie  very  far  apart,  but 
between  these  runs  somewhere  a  normal  line  or  "  line  of 
safety,"  which  is  occupied  by  those  individuals  which  may 
be  regarded  as  the  type  of  the  species.  The  doctrine  how- 
ever, of  the  evolution  of  species  by  natural  selection  de- 
mands our  assent  to  the  belief  that  the  variability  of  a  .species 
is  indefinite. 

6.  The  theory  of  the  evolution  of  species  by  natural  selec- 
tion implies  of  necessity  that  one  species  can  only  be  con- 
verted into  another  through  the  medium  of  a  great  number  of 
successive  forms,  graduating  into  one  another,  each  member 
of  the  series  diftering  from  its  immediate  neighbours  in 
but  minute  characters.  If,  therefore,  any  existing  species 
has  descended  from  any  pre-existing  species,  there  must  at 
one  time  have  existed  between  the  two  species  a  graduated 
series  of  intermediate  forms.  When  we  consider  the  enor- 
mous number  of  living  animals  and  plants,  and  the  still 
more  enormous  number  of  extinct  forms  which  we  know,  or 
may  infer,  to  have  existed  in  past  time,  it  becomes  clear — 
if  evolution  be  true — that  the  number  of  minutely  inter- 
mediate forms  must  have  been  incalculably  great  We 
have  therefore  the  clear  right  to  expect  that  Palaeontology 
should  reveal  to  us  such  intermediate  forms,  amongst  the 
vast  series  of  fossil  remains  with  which  we  are  acquainted. 
We  cannot,  however,  in  any  case  point  to  such  forms.  It 
is  quite  true  that  there  are  many  instances  in  which  fossil 
animals  may  be  regarded  as  intermediate  forms  between 
great  groups  of  living  forms,  as  missing  links  in  the 
zoological  chain.     Such  intermediate  forms,  however,  are 


142  ELEMENTS  OF  BIOLOGY. 

invariably  sharply  separated  from  the  forms  which  they 
connect ;  and  no  case  is  yet  known  to  us,  even  taking  the 
Tertiary  period  alone,  in  which  we  can  point  to  a  graduated 
series  of  intermediate  forms,  by  which  one  well-marked 
species  can  be  shown  to  pass  into  another  equally  well- 
marked  species. 

7.  The  changes  in  the  life  of  the  globe  revealed  to  us  by 
geology  are  so  vast  and  so  numerous  that  the  imagination  is 
utterly  powerless  to  grasp  the  inconceivable  lapse  of  time 
required  for  the  bringing  about  of  these  changes  by  the 
tardy  action  of  natural  selection  alone.  Physical  geology 
teaches  us  that  geological  time  is  something  as  inconceiv- 
ably vast  as  astronomical  space ;  but  it  may  fairly  be 
doubted  if  the  utmost  lapse  of  time  required  by  the  phe- 
nomena of  physical  geology  can  be  regarded  as  more  than 
a  mere  drop  in  the  ocean,  as  compared  with  the  time  re- 
quired for  the  zoological  revolutions  indicated  by  the  study 
of  Palaeontology — if  these  revolutions  have  been  brought 
about  by  the  action  of  natural  selection.  It  can  hardly  be 
reasonably  asserted  that  the  time  necessary  for  such  biolo- 
gical changes  is  fixed  by  physical  geology  alone ;  and  that 
if  this  latter  informs  us  that  the  geological  changes  of  the 
earth  have  taken  place  within  a  given  limited  period,  then 
we  must  simply  change  our  beliefs  as  to  the  time  required 
for  the  conversion  of  one  species  into  another  by  natural 
selection.  This  certainly  appears  to  be  a  species  of  reason- 
ing in  a  circle.  The  very  essence  of  the  theory  of  "  Evolu- 
tion by  Natural  Selection  "  is  the  almost  entire  impossibility 
of  one  species  being  converted  into  another  otherwise  than 
by  an  extremely  slow  process,  during  which  a  vast  number 
of  generations  lived  and  died.  We  have  also,  upon  the 
doctrine  of  "  the  adequacy  of  existing  causes,"  certain 
definite  data  as  to  the  duration  of  species.  For  we  know 
that  many  existinespecies  have  lived  without  change  during 
what  may  justly  be  considered  a  very  vast  period  of  time. 
It  is  therefore  for  Evolution  to  say  how  long  a  period  is 


ORIGIN  OF   SPECIES.  I43 

required  for  the  biological  revolutions  which  we  know  to 
have  occurred  since  the  Luurentian  period ;  and  if  physical 
geology  or  astronomy  can  show  tlut  the  period  demanded 
is  too  great,  Evolution  will  hardly  evade  the  difficulty  by 
shortening  the  time  required  for  the  conversion  of  one 
species  into  another.  At  present,  however,  it  can  only  be 
said  that  whilst  physical  geology  does  not  absolutely  need 
the  time  demanded  by  the  theory  of  Evolution,  there  is 
nothing  in  the  facts  of  the  former  which  would  forbid  our 
yielding  to  the  requirements  of  the  latter.  There  are,  on 
the  other  hand,  good  grounds  to  be  drawn  from  other  de- 
partments of  ])hysical  science,  as  shown  by  Sir  William 
Thomson,  for  the  belief  that  the  period  which  has  elapsed 
since  the  introduction  of  life  upon  the  earth  is  much  below 
that  which  is  required  by  the  theory  of  evolution  by  natural 
selection. 


ij- 


»•■•»  -    f  ",.■      '4/% 


'      ■      .<■■'      '.    »'.    ''        ••:'*       ■*        1    ..(■•• 


;   .     ,    ■  '  ' ''  •     -"i     I        •  '  1  1 1  •  < 

'.ft,     •   :.  .  ■      ^'  ,:•   ■  :,  V    '  , 
•    •  t    !     .    f       I       • 


C    r 


CHAPTER    XIV. 


t,  ■) 


DISTRIBUTION     IN     SPACE.  '*"    , 

Under  the  general  term  of  "  Distribution"  come  all  the 
facts  concerning  the  external  or  objective  relations  of  ani- 
mals— that  is  to  say,  their  relations  to  the  external  conditions 
by  which  they  are  surrounded. 

The  geographical  distribution  of  animals  is  concerned  with 
the  determination  of  the  areas  within  which  every  species  of 
animal  is  at  the  present  day  confined.  Some  species  are 
found  almost  everywhere,  when  they  are  said  to  be  "  cos- 
mopolitan ; "  but,  as  a  rule,  each  species  is  confined  to  a 
limited  and  definite  area.  Not  only  are  species  limited  in 
their  distribution,  but  it  is  possible  to  divide  the  earth's  sur- 
face into  a  certain  number  of  geographical  regions  or  "  zoo- 
logical provinces,"  each  of  which  is  characterised  by  the 
occurrence  in  it  of  certain  associated  forms  of  animal  life. 
The  number  of  these  provinces  has  not  yet  been  universally 
agreed  upon,  and  it  is  unnecessary  here  to  enter  into  this 
subject  in  detail.  There  are,  however,  some  general  con- 
siderations which  may  be  briefly  alluded  to. 

The  geographical  distribution  of  land  animals  is  condi- 
tioned partly  by  the  existence  of  suitable  surroundings,  and 
partly  by  the  presence  of  barriers  preventing  migrations. 
Thus,  certain  contiguous  regions  might  be  equally  suitable 
for  the  existence  of  the  same  animals,  but  they  might  belong 


■-  ( - 


DISTRIBUTION   IN  SPACE.  I45 

to  different  zoological  provinces,  if  separated  by  any  impass- 
able barrier,  such  as  a  lofty  chain  of  mountains.  Owing  to 
their  power  of  flight,  the  geographical  distribution  of  birds 
is  much  less  limited  than  that  of  mammals;  and  many  migra- 
tory birds  may  be  said  to  belong  to  two  zoological  provinces. 
In  spite  of  their  powers  of  locomotion,  however,  birds  arc 
limited  by  the  necessities  of  their  life  to  definite  areas,  and 
a  zoological  province  may  be  marked  b)  its  birds  just  as 
well  as  by  its  quadrupeds*. 

The  geographical  distribution  of  an  animal  at  the  present 
day  by  no  means  necessarily  coincides  with  its  former  ex- 
tension in  space.  Many  species  are  known  which  now 
occupy  a  much  more  restricted  area  than  they  did  formerly, 
owing  to  changes  in  climate,  the  agency  of  man,  or  other 
causes.  Similarly,  there  are  species  whose  present  area  is 
much  wider  than  it  was  originally.  ^.,  »   .% 

Zoological  provinces  must  always  have  existed  ;  but  those 
of  the  present  day  by  no  means  correspond  with  those  of 
former  periods  of  the  earth's  history,  but  are,  on  the  con- 
trary, of  comparatively  recent  origin. 

As  regards  the  Mammals,  the  same /orms  are  found  occu- 
pying the  same  regions  in  the  later  Tertiary  period  as  they 
do  at  present  j  but  the  species'  are  different.  The  distribu- 
tion, therefore,  of  certain  groups,  dates  back  to  a  period  an- 
terior to  the  appearance  of  the  now  existing  species  of  the 
same  groups.  Thus,  to  take  a  single  example,  South  Ame- 
rica at  the  present  day  has  amongst  its  many  peculiar  animals 
none  more  characteristic  than  the  Sloths  and  Armadillos 
{Edentata).  In  late  Tertiary  time,  however.  Edentate  ani- 
mals were  equally  characteristic  of  the  South  American 
fauna,  though  none  of  the  living  species  then  existed.  Thus, 
the  modern  Sloths  are  represented  by  the  gigantic  Me^at/te- 
rtumj  Mylodofiy  and  Megalonyx^  and  the  little  armour-plated 
Armadillos  find  their  ancient  representative  in  the  colossal 
Glyptodon.  It  is  to  be  remembered,  however,  that  the  law 
thus  indicated  holds  good  for  the  later  Tertiary  period  only, 


146  ELEMENTS  OF  BIOLOGY. 

and  does  not  apply  in  any  manner  that  admits  of  being 
traced  to  early  geological  epochs.  The  general  result  of 
this  law  is,  that  existing  zoological  provinces  are  in  some 
cases  older  than  the  species  by  which  they  are  now  char- 
acterised. 

The  vertical  Qtx  ^a/Zr^w^'/r/Vw/ distribution  of  animals  relates 
to  the  limits  of  depth  within  which  each  marine  species  is 
confined.  In  many  cases  it  is  found  that  marine  animals 
occupy  defmite  bathymetrical  zones,  existence  being  impos- 
sible, or  at  any  rate  difficult,  at  depths  greater  or  less  than 
those  comprised  within  the  limits  of  the  zone  which  each 
inhabits.  In  accordance  with  the  facts  at  that  time  known, 
naturalists  formerly  accepted  the  following  four  bathymetrical 
zones,  as  being  characterised  each  by  its  peculiar  fauna : — 

1.  The  Littoral  zone,  or  the  tract  between  tide-marks. 

2.  The  Laminarian  zone,  from  low  water  to  15  fathoms. 

3.  The  Coralline  zone,  from  15  to  50  fathoms. 

4.  The  Deep-sea  coral  zone,  from  50  to  100  fathoms  or 
more. 

Beyond  a  depth  of  something  between  100  and  200  fa- 
thoms it  was  formerly  believed  that  marine  life  did  not  ex- 
tend. Recent  researches,  however,  especially  those  by  Drs 
Carpenter  and  Wyville  Thcfmson  and  Mr  Gwyn  Jeffreys, 
have  greatly  modified  the  above  generalisation,  and  have  led 
to  the  establishment  of  conclusions  of  the  greatest  import- 
ance and  interest.  The  value  of  the  Littoral  zone,  or  the 
tract  between  tide-marks,  as  a  marine  province,  has  not  been 
affected  by  these  discoveries,  but  the  importance  of  the 
others  has  been  greatly  reduced  ;  and  we  might  well  adopt 
the  views  of  Mr  Gwyn  Jeffreys,  and  consider  that  there  are 
but  two  chief  bathymetrical  zones,  the  littoral  and  the  sub- 
marine. 

The  next  important  point  which  has  been  brought  to 
light  is,  that  life  extends  to  all  depths  in  the  ocean,  marine 
animals  having  been  dredged  in  abundance  from  a  depth 
of  2300  fathoms,  or  not  far  short  of  three  miles.     If,  there- 


DISTRIBUTION   IN   SPACE.  147 

fore,  we  are  to  retain  the  four  zones  above  mentioned,  we 
must  now  add  to  these  a  fifth  or  Abyssal  zone,  extending 
from  loo  fathoms  up  to  at  least  2500  fathoms,  and  doubt- 
less really  extending  to  all  depths  in  the  ocean. 

The  most  important  result,  however,  of  these  inquiries  is 
the  discovery  of  the  fact  that,  beyond  a  very  limited  depth, 
the  distribution  of  marine  animals  is  conditioned,  not  by  the 
depth  of  the  water,  but  by  its  temperature.  Thus  the  bathy- 
w^/r/Va/ distribution  is  in\\y  di  thcrmopnetricai  owt.  Similar 
forms,  namely,  are  found  inhabiting  areas  in  which  the 
bottom-temperature  is  the  same,  wholly  irrespective  of  the 
depth  of  water.  It  may  happen,  therefore,  that  two  dis- 
tinct faunae  may  inhabit  contiguous  areas  of  the  sea-bottom, 
and  may  be  even  sharply  marked  off  from  one  another,  as 
when  one  area  is  swept  by  a  warm  current,  v.hilst  a  neigh- 
bouring area  has  ils  f  emperature  lowered  by  a  cold  current. 

The  conditions  under  which  the  animals  of  the  deep  sea 
live,  are  so  different  to  those  to  which  the  inhabitants  of 
shallow  waters  are  subjected,  that  a  few  remarks  upon  this 
Subject  may  advantageously  be  added  here. 

It  was  formerly  believed  that  the  pressure  of  the  water  at 
great  depths  would  be  so  enormous  as  to  preclude  the  pos- 
sibility of  life  being  present.  This,  however,  is  a  fallacy ; 
since  the  internal  pressure  of  any  body  immersed  in  a  fluid, 
and  admitting  fluid  into  its  interior,  is  in  all  cases  the  exact 
equivalent  of  the  external  pressure.  In  other  words,  marine 
animals  are  in  this  respect  in  the  same  position  as  an  un- 
corked bottle  sunk  at  the  bottorr.  of  the  sea.  Whatever  the 
depth  may  be,  there  is  no  pressure  upon  the  sides  of  the 
bottle,  because  the  pressure  of  the  water  outside  the  bottle 
is  exactly  neutralised  by  the  pressure  of  the  water  in  its 
interior. 

In  the  second  place,  it  is  a  well-known  generalisation 
that  animals  are,  mediately  or  immediately,  dependent 
upon  plants  for  their  subsistence.  Plants,  however,  can- 
not exist  unless  supplied  with  solar  light,   and  there  is 


148  ELEMENTS  OF   BIOLOGV. 

reason  to  conclude  that  the  sun's  rays  can  at  most  but 
penetrate  to  a  depth  of  a  few  hundred  feet  below  the  sur- 
face of  the  sea.  In  the  absence,  therefore,  of  any  positive 
knowledge,  it  was  a  justifiable  conclusion  that  animal  life 
could  not  extend  to  very  great  depths  in  the  ocean,  since 
vegetable  life  would  of  necessity  be  absent.  In  the  deep 
sea,  however,  we  find  an  assemblage  of  animals,  not  essen- 
tially different  from  those  of  shallow  seas,  living  without,  or 
almo'jt  without,  vegetable  life  of  any  kind.  A  few  micro- 
scopical plants  there  may  possibly  be  ;  but  unquestionably 
there  is  nothing  that  could  for  one  moment  be  regarded  as 
supplying  vegetable  food  to  any  considerable  number  of 
animals.  The  question  then  arises,  How  do  these  animals 
support  existence?  Some,  of  course,  feed  upon  the  others  ; 
but,  equally  of  course,  this  must  have  a  limit ;  and  there 
must  be  some  which  have  the  means  of  obtaining  food  in 
some  other  manner.  Two  explanations  have  been  put 
forward  to  account  for  this  singular  fact.  On  the  one  hand, 
it  has  been  thought  that  some  01  the  deep-sea  animals 
might  perhaps  have  the  power  possessed  by  plants,  of 
taking  inorganic  substances  from  the  surrounding  medium, 
and  building  up  these  into  the  matter  of  life.  This  theory, 
if  provable,  would  be  all  th(  t  is  needed;  because  then,  in 
point  of  fact,  some  of  the  animals  of  the  deep  sea,  as 
regards  their  mode  of  feeding,  would  be  really  plants,  and 
thus  the  balance  of  organic  nature  would  be  maintained  in 
equilibrium.  This,  however,  is  a  mere  hypothesis  ;  and  it 
has  been  shown,  on  the  other  hand,  that  the  sea-water  at 
great  depths  holds  in  solution  a  very  much  larger  propor- 
tion of  organic  matter  than  is  nonnaliy  present ;  so  that  it 
may  practically  be  regarded  as  a  very  dilute  sot/^.  It  has 
therefore  been  suggested,  with  great  probability,  that  the 
lower  forms  of  life  in  these  abysses  can  support  'ife  solely 
upon  this  dissolved  and  soluble  organic  matter.  , 

Thirdly,  it  might  have  been  reasonably  anticipated  that 
the  water  at  great  depths  in  the  ocean  would  have  been 


DISTRIliUTION   IN   SPACE.  I49 

devoid  of  the  oxygen  necessary  for  the  support  of  animal 
life.  The  sea-water  is  rcainly  oxygenated  by  the  agitation 
of  the  waves,  and  this  extends  but  to  a  very  limited  depth 
below  the  surface.  It  is  now  known,  however,  that  the 
depth;  of  the  ocean,  though  tranquil  and  undisturbed  by 
storms,  are  nevertheless  renovated  by  a  vast  and  com- 
plex system  of  oceanic  currents.  In  this  way  the  oxy- 
genated life-supporting  surface-water  is  being  constantly 
transferred  from  the  fiice  of  jthe  deep  to  take  the  place  of 
the  airless  strata  of  the  abysses,  from  which  the  oxygen 
has  been  removed  by  the  agency  of  living  beings. 

Lastly,  we  have  to  consider  how  the  animals  of  the  deep 
sea  manage  to  exist  in  the  absence  of  light.  For  plant-life 
light  is  absolutely  essential ;  and  though  it  is  possible  for 
finimals  to  exist  in  total  darkness,  the  cases  in  which  this 
occurs  are  few,  and  the  absence  of  light  is  generally  ac- 
companied by  the  loss  of  organs  of  vision.  As  a  general 
rile,  however,  light  is  ail-important  to  animal  life  in  its 
higher  developments,  if  only  for  the  reason  that  without 
light  the  predaceous  animals  could  not  see  to  capture  their 
prey.  Without  dogmatising  as  to  the  depth  below  the 
surface  to  Avhich  light  may  penetrate,  it  seems  certain  that 
Egyptian  darkness  must  prevail  at  all  depths  below  a  few 
hundred  fathoms.  This  would  at  once  account  for  the 
absence  in  the  deep  sea  of  all  vegetable  life,  with  the 
exception  of  such  microscopic  plants  as  most  probably  live 
at  or  near  the  surface,  and  only  fall  to  the  bottom  when 
dead.  Nevertheless,  in  the  face  of  this,  we  find  animals 
living  at  a  depth  of  more  than  two  thousand  fathoms  with 
perfect  and  well-developed  eyes,  as  perfect  as  the  organs  of 
vision  possessed  by  animals  living  in  illuminated  regions. 
It  has  been  suggested  by  Sir  Charles  Lyell,  as  an  explanation 
Oi  ,.iis  fact,  that  the  deep-sea  a^.  nials  are  enabled  to  see  by 
their  own  phosphorescence.  It  is  certain  that  many  of  them 
phosphoresce  brilliantly,  and  in  the  absence  of  any  odier 
source  of  light  it  seems  almost  certain  that  they  must  owe 


150  ELEMENTS  OF  BIOLOGY. 

their  means  of  vision  tc  this  property.  If  this  be  the  case, 
we  have  here  one  of  the  most  wonderful  adaptations  in  the 
whole  range  of  animated  nature,  by  which  life  is  rendered 
possible  amidst  the  most  apparently  hostile  conditions. 
Good  authorities,  however,  are  indisposed  to  accept  this 
view,  and  some  other  explanation  of  the  facts  may  yet  be 
found. 


CHAPTER    XV. 


DISTRIBUTION     IN     TIME. 


All  the  facts  which  concern  the  existence  of  living  beings 
in  past  periods  of  the  earth's  history  come  under  the  general 
head  of  "  Distribution  in  Time." 

The  laws  of  distribution  in  time  are,  however,  from  the 
nature  of  the  case,  less  perfectly  known  than  are  the  laws 
of  lateral  or  vertical  distribution,  since  these  latter  concern 
beings  which  we  are  able  to  examine  directly.  The  follow- 
ing are  the  chief  facts  which  it  is  necessary  for  the  student 
to  bear  in  mind : — 

1.  The  rocks  which  compose  the  crust  of  the  earth  have 
been  formed  at  successive  periods,  and  may  be  roughly 
divided  into  aqueous  or  sedimentary  rocks,  and  igneous 
rocks. 

2.  The  igneous  rocks  are  produced  by  the  agency  of  heat, 
are  mostly  unstratijied  (i.  e.,  are  not  deposited  in  distinct 
layers  or  strata),  and,  with  few  exceptions,  ar^  destitute  of 
any  traces  pf  past  life. 

3.  The  sedimentary  or  aqueous  rocks  owe  their  origin  to 
the  action  of  water,  are  stratified  (/.  e.,  consist  of  separate 
layers  or  strata),  and  mostly  exhibit  "  fossils  " — that  is  to 
say,  the  remains  or  traces  of  animals  or  plants  which  were 
in  existence  at  the  time  when  the  rocks  were  deposited. 

4.  The  series  of  aqueous  rocks  is  capable  of  being  divided 


152  ELEMENTS  OF   BIOLOGY. 

into   a   number  of  definite  groups   of   strata,    which   are 
technically  called  "  formations." 

5.  Each  of  these  definite  rock-groups,  or  "  formations,"  is 
characterised  by  the  occurrence  of  an  assemblage  of  fossil 
remains  more  or  less  peculiar  and  confined  to  itself. 

6.  The  majority  of  these  fossil  fonns  are  "  extinct ; "  that 
is  to  say,  they  do  not  admit  of  being  referred  to  any  species 
at  present  existing. 

7.  No  fossil,  however,  is  known,  which  cannot  be  referred 
to  one  or  other  of  the  primary  subdivisions  of  the  Animal 
Kingdom,  which  are  represented  at  the  present  day. 

8.  When  a  species  has  once  died  out  it  never  reappears. 

9.  The  older  the  formation,  the  greater  is  the  divergence 
between  its  fossils  and  the  animals  and  plants  now  existing 
on  the  globe.  ■ 

10.  All  the  known  formations  are  divided  into  three  great 
groups,  termed  respectively  Palaeozoic  or  Primary,  Mesozoi : 
or  Secondary,  and  Kainozoic  or  Tertiary. 

The  Palaeozoic  or  Ancient-life  period  is  the  oldest,  and 
is  characterised  by  the  marked  divergence  of  the  life  of  the 
period  from  all  existing  forms. 

In  the  Mesozoic  or  Middle-life  period,  the  general  facics 
of  the  fossils  approaches  more  nearly  to  that  of  our  existing 
fauna  and  flora;  but — with  very  few  exceptions  —  the 
characteristic  fossils  are  all  specifically  distinct  from  all  exist- 
ing forms. 

In  the  Kainozoic  or  New-life  period,  the  approximation 
of  the  fossil  remains  to  existing  living  beings  is  still  closer, 
and  some  of  the  forms  are  now  specifically  identical  with 
recent  species ;  the  number  of  these  increasing  rapidly  as 
we  ascend  from  the  lowest  Kainozoic  deposit  to  the  Recent 
period. 


DISTRIBUTION   IN   TIME. 


153 


Ideal  Section  of  the  Crust  of  the  Earth. 

Fig.  39. 


rost-Tcrtiary  and  Recent* 
Pliocene. 


■'  -•'■  ■( 

J 

•  ■  i 

»   K'.,, 

.   f 

'  br-x 

;*•  ■-: 

-.-'■•'■.vy, 

154  ELEMENTS   OF   BIOLOGY. 

Subjoined  is  a  table  giving  the  more  important  subdivi- 
sions of  the  three  great  geological  periods,  commencing 
with  the  oldest  rocks  and  ascending  to  the  present  day. 
{See  fig.  39.) 

I.  Paleozoic  or  Primary  Rocks. 

1.  Laurentian.     (Lower  and  Upper.) 

2.  Cambrian.   (Lowerand  Upper,  with  Huronian  Rocks?) 

3.  Silurian.     (Lower  and  Upper.) 

4.  Devonian,  or  Old  Red  Sandstone.  (Lower,  Middle, 
and  Upper.) 

5.  Carboniferous.  (Mountain-limestone,  Millstone  Grit, 
and  Coal-measures.) 

6.  Permian.  ( =  The  lower  portion  of  the  New  Red 
Sandstone.) 

II.  Mesozgic  or  Secondary  Rocks. 

7.  Triassic  Rocks.  (Bunter  Sand  lutein,  or  Lower  Trias; 
Muschelkalk,  or  Middle  Trias;  Keuper,  or  Upper  Trias.) 

8.  Jurassic  Rocks.  (Lias,  Inferior  Oolite,  Great  Oolite, 
Oxford  Clay,  Coral  Rag,  Kimmeridge  Clay,  Portland  Stone, 
Purbeck  beds.) 

9.  Cretaceous  Rocks.  (Wealden,  Lower  Greensand, 
Gault,  Upper  Greensand,  White  Chalk,  Maestricht -beds.) 

III.  Kainozoic  or  Tertiary  Rocks. 

10.  Eocene.     (Lower,  Middle,  and  Upper.) 

11.  Miocene.     (Lower  and  Upper.) 

12.  Pliocene.     (Older  Pliocene  and  Newer  Pliocene.) 

13.  Post-tertiary.     (Post-pliocene  and  Recent.) 

Contemporaneity  of  Strata. — When  groups  of  beds 
in  different  regions  contain  the  same  fossils,  or  rather  an 
assemblage  of  fossils  in  which  many  identical  forms  occur, 
they  are  ordinarily  said  to  be  "contemporaneous;"  that 
is  to  say,  they  are  ordinarily  supposed  to  have  been  formed 


DISTRIBUTION   IN   TIME. 


155 


at  the  same  period  in  the  history  of  the  earth,  and  belong 
to  the  same  geological  epoch. 

This  statement,  however,  can  only  be  received  with  some 
important  qualifications.  Beds  containing  the  same  specific 
forms  are  often  so  widely  removed  from  one  another  in 
point  of  distance,  and  occur  at  so  many  different  points  of 
the  earth's  surface,  that  it  becomes  inconceivable  that  they 
are  "  contemporaneous "  in  the  literal  sense  of  this  term. 
Such  a  supposition  would  imply  an  ocean  not  only  more 
widely  extended  but  presenting  more  uniform  conditions 
than  any  with  which  we  are  at  the^  present  day  acquainted. 
Besides,  we  know  that  strictly  contemporaneous  beds  would 
rarely  contain  exactly  the  same  species  of  fossils.  Thus,  if 
we  could  examine  the  bed  of  the  Atlantic,  we  should  un- 
doubtedly find  it  occupied  by  a  series  of  deposits  which 
would  be  "  contemporaneous  "  in  the  strictest  sense  of  the 
term,  but  they  would  neither  have  the  same  mineral  char- 
acters, nor  contain  the  same  or  even  nearly-related  fossils. 
Some  of  the  deposits,  for  example,  would  consist  of  chalky 
beds,  crowded  with  Foraminifera,  Siliceous  Sponges,  Crinoids, 
and  Sea-urchins.  Others  would  be  composed  of  sand  and 
mud,  and  would  contain  the  remains  of  Arctic  shells. 
Others,  again,  would  have  the  characters  of  shore-deposits, 
and  would  yield  the  remains  of  littoral  animals.  If  this  be 
the  case  with  a  single  ocean,  such  as  the  Atlantic,  still  more 
is  it  the  case  when  we  consider  all  the  oceans  of  the  globe, 
the  deposits  of  which  are  nevertheless  contemporaneous,  in 
the  sense  that  they  have  been  formed  at  the  same  time. 

Contemporaneous  beds,  then,  if  separated  from  one 
another  in  point  of  distance,  are  by  no  means  likely  to 
contain  the  same  species  of  lossils.  We  are  thus  driven  to 
seek  for  another  explanation  of  the  fact  that  specifically 
identical  fossils  are  often  found  in  formations  very  widely 
removed  from  one  another.  The  true  explanation  of  this 
fact  is  to  be  sought  in  the  phenomenon  of  **  migration." 
If  we  imagine  a  given  assemblage  of  animals  to  be  inhabit- 


156  ELEMENTS  OF  BIOLOGY. 

ing  a  given  area  of  the  sea-bottom,  and  \vc  ?uppose  the  con- 
ditions of  that  area  to  be  changed  for  the  worse,  either  by 
an  elevation  of  the  sea-bottom  or  from  any  other  cause,  a 
migration  of  the  fauna  will  be  set  on  foot.  The  locomotive 
animals  will  shift  their  quarters  in  search  of  some  other  area 
in  which  the  conditions  are  more  favourable  to  their  exist- 
ence. As  sedentary  animals  have  almost  universally  loco- 
motive young,  we  may  from  this  poi'  i  of  view  regard  all  the 
animals  of  such  an  area  as  capable  of  migrating.  A  general 
migration  of  the  fauna  of  the  area  will  commence,  and  in 
this  way  some  of  the  species  of  the  area  will  be  transferred 
to  another  area.  By  a  repetition  of  this  process  the  same 
species  may  ultimately  com.e  to  inhabit  an  area  removed  by 
a  hemisphere  from  its  original  habitat ;  and  in  this  way  the 
same  species  may  present  itself  in  beds  at  the  most  distant 
parts  of  the  earth's  surface. 

It  is  quite  clear,  however,  from  the  above,  that  the  iden- 
tity of  fossils  in  widely  distant  strata,  is,  upon  the  whole,  a 
proof  that  the  beds  in  question  are  not  strictly  contempo- 
raneous. A  migration  is  a  work  of  time,  and  one  of  the  two 
sets  of  beds  must  obviously  and  necessarily  be  younger 
than  the  other  by  the  period  consumed  in  the  migration. 
Still  the  interval  between  two  such  sets  of  beds  would  not 
be  long,  geologically  speaking,  and  both  groups  of  strata 
would  belong  approximately  to  the  same  geological  horizon. 
If,  therefore,  we  still  apply  the  name  of  "  contemporaneous  " 
to  beds  which  contain  the  same  fossils  but  are  widely 
separated  from  one  another  in  point  of  distance,  we  must 
do  so  on  the  clear  understanding  that  the  term  must  be 
taken  in  a  wider  and  looser  sense  than  that  in  which  it  is 
ordinarily  employed.  •  ,    ..:.::.... 

Geological  Continuity. — The  entire  series  of  Stratified 
or  Fossiliferous  rocks,  as  before  remarked,  admits  of  a  natu- 
ral division  into  a  certain  number  of  definite  "  rock-groups  " 
or  "  formations,"  each  of  which  is  characterised  by  a  peculiar 
and  distinctive  assemblage  of  fossils,  constituting  the  "  life  " 


DISTRIliUTION   IN   TIME. 


157 


of  the  period  in  which  the  formation  was  deposited.  It  is 
a  matter  of  importance  to  understand  clearly  how  far  these 
subdivisions  are  natural,  and  what  value  we  may  attach  to 
them.  The  older  and  very  natural  view  held  that  the  close 
of  each  formation  was  signalised  by  a  general  destruction  of 
all  the  forms  of  life  characteristic  of  the  period,  and  that 
the  commencement  of  each  new  formation  was  accompanied 
by  the  creation  of  a  number  of  new  forms.  On  the  more 
modern  view,  it  is  held  that  the  great  formations,  and  many 
of  the  minor  subdivisions,  are  separated  by  longer  or  shorter 
lapses  of  time  not  represented  by  any  deposition  of  rock  in 
the  area  where  the  formations  in  question  are  in  contact. 
Upon  this  view  we  have  to  admit  that  what  we  call  the 
great  "  fomiations "  are  purely  artificial  divisions  rendered 
possible  by  the  gaps  in  our  knowledge  only ;  and  that  if  we 
had  a  complete  series  of  rock-groups,  we  could  have  no 
such  lines  of  demarcation. 

It  is  unnecessary  to  consider  here  why  it  is  that  we  can 
never  hope  to  find  a  complete  series  of  intermediate  rock- 
groups  by  which  any  two  great  formations  might  be  linked 
together.  It  is  sufficient  to  say  that  we  may  well  have  the 
strong  conviction  that  such  intermediate  deposits  have  at 
one  time  existed,  or  must  still  exist,  whilst  there  are  per- 
fectly valid  reasons  for  the  belief  that  we  can  never  know 
more  than  fragments  of  them. 

Most  modern  geologists,  then,  would  hold  that  there  is  a 
geological  "  continuity,"  such  as  we  see  in  other  departments 
of  nature.  There  can  have  been  in  reality  no  break  in  the 
great  series  of  stratified  deposits ;  but  there  must  have  been 
a  complete  "continuity"  of  life  and  deposition  from  the 
Laurentian  period  to  the  present  day.  There  was,  and  could 
h$ve  been,  no  such  continuity  in  any  one  area;  but  it  is 
inconceivable  that  the  chain  should  have  been  snapped  at 
one  point  and  again  taken  up  at  another  wholly  different 
one.  The  links  of  the  chain  may,  indeed  must,  have  been 
forged  in  different  places,  but  its  continuity  must  neverthe- 
8 


158  ELEMEN'TS  OF   BIOLOGY. 

less  have  remained  unbroken.  From  this  point  of  view 
there  would  be  little  impropriety  in  saying  that  we  are  still 
living  in  the  Silurian  period  ;  but  we  could  only  say  so  in  a 
very  limited  sense.  Most  geologists  would  freely  admit 
that  there  must  in  nature  have  been  an  actual  continuity  of 
the  great  geological  periods.  Nevertheless  it  remains  cer- 
tain that  we  can  never  dispense  with  the  division  of  the 
stratified  series  into  definite  rock-groups  and  life-periods. 
We  can  never  hope  to  discover  all  the  lost  links  of  the 
geological  chain  ;  and  the  great  formations  are  likely  ever  to 
remain  separated  by  more  or  less  pronounced  physical  or 
palaeontological  breaks,  or  both  combined.  The  utmost  we 
can  at  present  do  is  to  arrive  at  the  conviction  that  the 
lines  of  demarcation  between  the  great  formations  only 
mark  gaps  in  our  knowledge,  and  that  there  can  be  truly  no 
hiatus  in  the  long  series  of  fossiliferous  deposits. 

Imperfection  of  the  Pal^ontological  Record. — 
As  has  been  just  pointed  out,  the  series  of  the  stratified 
formations  is  to  be  regarded  as  an  imperfect  one,  in  which 
many  links  are  missing.  The  causes  of  this  "  imperfection 
of  the  geological  record,"  as  it  has  been  termed  by  Darwin, 
are  various ;  but  the  most  important  ones  are  our  as  yet 
limited  knowledge  of  vast  areas  of  the  earth's  surface,  the 
process  of  denudation,  and  the  fact  that  many  of  the  miss- 
ing groups  are  buried  beneath  other  deposits,  whilst  more 
than  half  of  the  superficies  of  the  globe  is  hidden  from  us 
by  the  waters  of  the  sea.  The  imperfection  of  the  geological 
record  necessarily  implies  an  equal  imperfection  of  the 
"  palaeontological  record  ; "  but,  in  truth,  the  record  of  life 
is  far  more  imperfect  than  the  mere  physical  series  of  de- 
posits. The  following  are  the  chief  causes  of  the  imperfec- 
tion of  the  palaeontological  record : —  * 

I.  In  the  first  place,  even  if  the  series  of  the  stratified 
deposits  had  been  preserved  to  us  in  its  entirety,  and  we  could 
point  to  sedimentary  accumulations  belonging  to  every 
period  in  the  earth's  history,  there  Avould  still  have  been 


DISTRIBUTION   IN   TIME.  1 59 

enormous  gaps  in  the  palaeontological  record,  owing  to  the 
different  facilities  with  which  different  animals  may  be  pre- 
served as  fossils.  It  is  impossible  here  to  enter  at  length 
into  this  subject ;  but  there  tc  obvious  reasons  why  certain 
groups  of  animals  should  never  be  found  as  fossils,  or  should 
at  best  be  but  sparingly  and  imperfectly  represented. 
Thus,  many  animals  are  entirely  soft-bodied,  destitute  of 
hard  parts  capable  of  being  preserved  in  a  fossil  condition, 
and  we  can  therefore  never  obtain  evidence  of  the  past 
existence  of  such  forms,  though  this  affords  no  presump- 
tion that  they  were  non-existent  at  any  given  period. 
Again,  most  sedimentary  deposits  have  been  laid  down  in 
the  sea,  and  contain,  therefore,  the  remains  of  marine 
animals,  if  not  exclusively,  at  any  rate  in  preponderating 
numbers.  Marine  groups  of  animals  are  therefore  much 
more  likely  to  be  preserved  than  the  inhabitants  of  lakes  or 
rivers.  Lastly,  almost  all  sedimentary  accumulations  have 
been  deposited  in  water,  whether  salt  or  fresh;  and  it  follows 
from  this  that  the  preservation  of  terrestrial  or  aerial  animals 
must  always  have  been  of  an  accidental  nature,  so  to  speak, 
depending  upon  the  chance  falling  of  such  an  animal  into 
water  where  sediment  was  being  accumulated.  It  is  only  in 
the  rare  cases  in  which  an  old  land-surface  has  been  pre- 
served to  us  that  we  meet  with  the  remains  of  such  animals 
as  fossils  properly  belonging  to  the  deposit  in  which  they 
occur. 

2,  In  the  second  place,  as  shown  by  the  imperfection  of 
the  geological  record,  there  are  vast  periods  in  the  earth's 
history  which  are  not  known  to  us  to  be  represented  by 
any  deposits.  This  of  necessity  leads  to  our  being  totally 
ignorant  of  the  life  of  these  same  periods.  As  already 
remarked,  we  can  never  expect  wholly  to  fill  up  these 
periods  of  "  unrepresented  time "  by  the  discovery  of  new 
deposits,  and  our  palaeontological  knowledge  will  therefore 
ever  remain  more  or  less  interrupted  and  incomplete. 

3.  In  the  third  place,  we  can  seldom  or  never  point  to 


l6o  ELEMENTS  OF   lUOI.OGY. 

more  than  one  or  two  classes  of  the  deposits  which  must 
have  been  formed  in  every  great  period.  We  n^y  have 
the  deep-soa  deposits  of  the  period,  or  the  littoral  accumu- 
lations, or  the  sediments  which  were  laid  down  in  its  rivers 
and  lakes ;  but  we  very  seldom,  if  ever,  obtain  all  of  these. 
We  can  therefore  rarely  expect  to  acquire  a  complete 
knowledge  of  even  the  acjuatic  animals  alone  of  any  period. 
4.  Lastly,  we  have  every  reason  to  believe  that  the  life  of 
vast  periods  of  the  earth's  history  will  ever  remain  to  us 
wholly,  or  almost  wholly,  unknown,  in  consequence  of  the 
fact  that  the  deposits  of  these  periods  have  been  subjected 
to  such  change  that  all  traces  of  their  contained  fossils  have 
been  destroyed 


.•  :<■■.. 


INDEX. 


iBiooKNMis,  127;  experimonU  of  Bu- 

tiau  on,  131-133.  ^ 

AbystiHl  rone,  147. 
AerogencB,  42. 
Actinozoa,  37. 

Air,  aa  a  condition  of  life,  14. 
Albumen,  67,  69. 
Alternation  of  generations,  104,    112. 

11. M,  125.  I         .        •, 

Anwba.O,  17,  28,  29,30. 
Amphibia,  41. 

Aniiloffy,  44. 

Anarthropoda,  38. 

Angiosponns,  43 ;  reproduction  of,  124- 
126. 

Animal  functions,  27,  77. 

Animals,  form  of,  20 ;  Internal  structure 
of,  21 ;  clieiiiical  composition  of,  21 : 
motor  power  of,  22;  food  of,  23:  re- 
spiration of,  24. 

Animals  ai:  :  rlnnts,  differences  between. 

19-25. 
Annelida,  38. 

Annuloida,  34;  definition  of,  37. 
Annulosa,  34;  deJinition  of,  38. 
Anophyta,  42. 

Aphides,  parthenogenesis  of,  114, 115. 
A  rachnuia,  89. 
Armadillos,  145. 
A  rlhropoda,  39. 
Ascidians,  cellulose  In,  21. 
AMHlmilation,  2,  85. 
Atrophy,  88. 

Bacteria,  14, 128, 132. 
Bathybius,  24. 

Batliyinetrical  distribution,  148. 
Bees,  parthenogenesis  of,  116,  116. 
Biology,  deflnition  of,  1. 
Bioplasm,  7,  70;  nature  of,  71;  move- 
ments of,  71. 
Brachiapoda,  35. 

Caeti,  51. 

Campanutaria,  107. 
Caserne,  67,  69. 


Cells,  72;  wall  of,  78;  cont«nt«  of,  74; 

il','"'","',."''  ^^J  nmltiplication  of,  75 
life  of,  79. 

Cellulose,  21,  69. 

Cephalopoda,  40. 

Chmtognatha,  39. 

Chemistry,  of  living  beingfl,  64,  65;  of 

animals,  65,  68;  of  vegetables,  68,  69. 
Chlorophyll.  22. 
Class,  deniiitlon  of,  62. 
ClasHiflcation,  66;  linear.  62. 
Clytia,  107. 

CoeUnUrata,  22,  84;  deflnition  of,  36. 
Conditions  of  life  in  the  deep  sHa.  147-150 
Contemporaneity  of  strnte,  154-166 
Continuity,  geological,  156.163. 
Correlation,  functions  of  {see  Relation! 
Correlation  of  growth,  64,  66. 
Crustacea,  39. 

Cryptogams.  42;  reproduction  of.  124. 
Cytogeneais,  75,  76. 

DARwiNTAir  theory,  138,  137;  objec- 
tions to,  139-143.  •' 

Dead  bodies,  chemical  composition  of 
4;  form  of,  6;  arrangement  of  parts 

Dead  and  living  bodies,  differences  be- 
tween. 2. 
Death,  15,  88, 

Deep  sea,  condition  of  life  in,  147-160. 
Desmlds,  22. 

Development,  89,  91 ;  Von  Baer'a  law  of. 
^  93 :  retrograde,  95.  ' 

Diatoms.  22.  ::  ^  i.  , 

Dicotyledons,  43. 
DictyogencB,  42. 
Difference*  between  different  organisms. 

Dimorphic  plants,  60. 

Distribution,  144;   in  spac  .  144;  geo- 

graphical  144-146 ;  bathymetrical.  14»- 

160;  in  time,  151-160. 

Eehinodermata,  87. 
Edentata,  distribution  of,  14B. 


1 62 


INDEX. 


Emliryology,  26. 

Endogencc,  42. 

Eiidiigenous  cell-multiplication,  76. 

Kpizoa,  95. 

Eu^jhorhice,  61. 

E-oliition,  theory  of,  04,  1.16;  vie'vs  of. 

Lamarck  on,  136;  viaws  of  Darwic  on, 

136. 

Family,  dofinition  of,  62. 

Ferns,  reproduction  of,  124-126. 

Fihiine,  67,  69. 

Fission,  98,  101 ;  of  Paramcecium,  101. 

P'issiparous  cell-multiplication,  76. 

F lustra,  20,  99,  100. 

Food,  of  plants,  2"  24 ;  of  animals,  23, 

24 ;  of  fungi,  23. 
Foraminifera,  6,  »,  12,  13,  71,  79,  PC,  99. . 
FuKotions,  physiological,  of  nniuials  and 

plants,   77-83;  of  nutrition,   27,   77, 

84-89;  of  reproduction,  27,  77,  97-118; 

of  plants,  113-126;  of  relation,  27,  77. 

Oasferopoda,  40 ;  young  of,  94. 
Geinraation,  98;   in  Foraminifera,  98, 

99 ;  of  Flustra,  99 ;  of  Uydra,  101 ; 

internal,  103. 
Gemmiparous  cell-multiplication,  76. 
Generation,  spontaneous,  127-133. 
Gen'rations,  alternation  of,  104, 112, 113, 

125. 
Genus,  definition  of,  61. 
Geographical  distribution,  144, 145. 
Geological  i!ontinuity,  156-158. 
Geological  distrihution,  laws  of,  151, 152. 
Geological  formations,  154;  periods,  152. 
Gephyrea,  39. 
Gluten,  69. 
GIy(!ogen,  21. 
Glyptodor.,  l45. 
G'>r.ijsomft,  107,  1'.  «. 
Qregarinida,  35. 

Heliconidce,  52. 

Histology,  26. 

Houiogeny,  49. 

Homology,  44;  serial,  '" ;  lateral,  43. 

Honioiilusy,  49,  51. 

IIomori)lMam,  60,  51. 

Hydra,  chloroi)hyll  i^i,  22;  gemmation 

of,  101 ;  individual!  .y  of,  102. 
Tlydraclinia,  104-107, 
Hydra-tuha,  110-112. 
Hyd'oid  zoo,  hytes,  20,  51. 
JI;fdvozoa,&„ 

lMPERrr:cTioN  of  the  Paleeontological 
record,  158-160. 

Individual,  98  ;  zoclngical,  1P2, 103. 

Infusoma  animalcules,  20,  22,36;  ap- 
pearance of,  in  organic  infusions,  12^. 

InsecU    39. 

JellyFishui,  108.  * 

Lameuibranciiiata,  id.        ..'''',''''^y'\"'-^. 


Legumine,  69. 

Life,  definition  of,  6  ;  physical  baiiis  of, 

6 ;  connection  of,    with  protoplasm, 

8-11 ;  connection  of,  with  organisation, 

11, 12. 
Light,  as  a  condition  of  life,  13. 
Linear  classiflcatiui  ,  impossibility  of,  62. 
Littoral  zone,  146. 
Living  bodies,  energy  of,  3 ;   chemical 

composition   of,  3 ;    arrangement   of 

parts  of,  4  ;  fori"  of,  6. 
Lucemarida,  deveiopment  of,  110. 

Mammalia,  42. 

Marsupials,  5i. 

Medusoids,  108,  109. 

Mc'jalonyx,  145. 

Megatherium,,  145.  ' 

Metagenesis,  113. 

Metamorphosis,  89-91. 

Migrations,  155,  156. 

Mimicry,  61-54. 

Molecules,  71 ;  of  organic  infusions,  128. 

Mollusca,  34  ;  definition  of,  39. 

Molluscoida,  20 ;  definition  of,  iO. 

Mimera,    i,  79. 

Monocotyledones,  42. 

Mor]iholiigical  type,  28,  .'53. 

Morphology,  delluition  of,  26. 

Myl'idon,  145. 

Myriapoda,  39.  ' 

N.VTiiRAL  selection,  137. 

Nitrogenous  compounds,  of  animals,  67; 

of  plants,  69. 
Non-nitrogenous  compounds,  of  animals, 

«6 ;  of  i)lauts.  68. 
Non-sexual  reproduction,  98.  • 

Nucleolus,  of  cells,  75. 
Nucleus,  of  cells,  74. 

Ordeu,  definition  of,  f  2.  . 

Organic  functions,  27,  77. 

Orgpuic  infusions,  development  of  living 

beings  in,  127-129. 
Organisation,  11. 
Ovum,  nature  of,  103,  113. 

PALiEONTOLOGiCAL  Tccoid,  impcrfcction 

of,  158-160. 
Paramoeciui.i,  fission  of,  101. 
Parthnnogeiesis,  113*  of  Aphides,  114; 

of£ees.  115. 
Periods,  geological,  152. 
Phasm,idce,  62. 
Phosphorescence,  of  deep-sea  animals, 

149. 
Physiology,  27. 
Pisces,  41. 
Plants,  form  of,  20;  internal  stnictnre 

of,  21 ;  chemical  composition  of,  21 ; 

motor  power  of,  22 ;  food  of,  23. 
Pollen,  122,  123. 
Pollen-tubes,  123. 
Polyzoa,  39,  61,  103. 
Proteids,  63. 


INDEX. 


163 


Protcine,  67. 

Pi(>toiis-animaloul6,  1?.. 

Prothalliia,  of  ferns,  124,  125. 

Protococcus,  20. 

Protophyta,  20. 

PnitoplaHni,  6,  7;   connection  of,  with 

lifo,  8 ;  living  and  dead,  10;  nature  of, 

71;  iiiovemeDts  of,  71. 
Protozoa,  20,  28  ;  definition  of,  85. 
Provisional  larvae,  92. 
Provisional  organs,  91. 
Proximate  compounds,  66. 
P.feudova,  114. 
Pteroiiods,  relations  of,  to  Gasteropods, 

Race,  definition  of,  59. 

Kegnuin  Protisticuni,  19. 

Kelation,  functions  of,  27. 

llelations  lietween  nutritive  and  genera- 
tive functions,  117,  118. 

Representative  forins,  51. 

Reproduction,  27,  97,  126;  sexual,  97; 
non-sexual,  98  ;  of  lost  parts,  98  ;  by 
gemmation  and  fission,  98-103 ;  rela- 
tions of,  to  nutrition,  117  ;  of  plants. 
119-126.  >        »       1  . 

Itcptilia,  41.  1 

Retrograde  development,  95. 
Ithizopoda,  35. 
llotijera,  tenacity  of  life  of,  15. 

Seclecida,  38. 

Sea-anemone,  30. 

Sea-mat,  20,  100. 

Selection,  natural,  187;  sexual,  137-139. 

Sexual  reproduction,  97. 

Sexual  selection,  137-139. 

Sloths,  145. 

Special  creation  of  species,  doctrine  of. 

135. 
Specialisation  of  functions,  28  33,  78. 


Species,  definition  of,  57-61 ;   origin  of. 

134-143.  •       *.         . 

Specific  centres,  136. 
Spongea,  20. 

Sptmtaneous  generation,  127-133. 
Starch,  21,  6G. 
Statoblastrt,  103. 
Stentor,  cJilorophyll  in,  22. 
Sub -kingdoms,  34-62;    definitions  of, 

35-42. 
Sugar,  69. 

Temperature,  as  a  condition  of  lif«, 

14. 
Thallophyta,  42. 
Transformation,  89,  90. 
Trimorphic  plants,  60. 
Trophozonie,  107,  120. 
Txinicata,  39. 

Unicellular  plants,  72,  78. 

Variety,  definition  of,  59. 

Vaucheria,  20,  21. 

Vegetative  functions,  27,  77. 

Vegetative  repetition,  47,  99. 

VerUhrata,  34,  48;  definition  of,  41. 

Vibrio  nea,  14, 128,  133. 

Vital  force,  9,  10,  11,  16.  80  ;  correlation 

with  physical  forces,  80-83. 
Vitality  {see  Lite). 
Volvox,  21. 
Von  Baer's  la,w  of  devolopmcnt,  93. 

Water,  as  a  condition  of  life,  15. 
Wlieel-auimaloules,  15. 

Yeast-plant,  72,  78,  79. 

Z061D,  102. 

Zoological  individual,  definition  of,  102. 

Zoological  provinces,  144. 


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